Add plugin support

@owensd convinced me that `plugin` is the wrong name.  We should call
@them Custom Tools, with an extension `.attool`.  This emphasizes that
@they are written for AT and provides something to google.

Replace bad diagnostic

We're supposed to be building simple, hackable tools but atbuild is
becoming more of a monolithic tool.  This patch aims to change that.

There are many reasons some feature should not be included in core:

1.  Core needs to be minimal so we can bootstrap it on new platforms;
    every new feature is a new burden

2.  Core needs to be x-platform but many features do not
    (see: xcode-emit, packageframework)

3.  Features may want different release frequency (see: xcode-emit) or
    don't want to coordinate with atbuild

4.  Features may be "not part of AnarchyTools" (Caroline) but still
    commonly used together

5.  I'm annoyed at upstream over the static linking issue, and thinking
    of ways to keep us from being that point of central failure that annoys
    somebody else someday

Therefore, we introduce the world's simplest plugin system designed to
move code out of core, or keep code out of core that doesn't need to be.

xcode-emit and Caroline will consume this API.  packageframework is a
good candidate for a plugin that might be moved out from core.

Documentation to follow

Don't print stack trace for user-visible subcommand errors

Add proper platform support

Resolve required overlays bug

Resolve #66

Fixes #65

Related to #36

Presently, we tend to enable platform-specific config with an overlay.
There are a variety of problems identified with this approach:

1.  There is no convention for which overlay to use for platform-
specific config.  This complicates the ecosystem.

2.  In general, a program is always compiled "for some platform" but in
practice a user may forget the necessary overlay.  `require-overlays`
can catch this misconfig, but A) it has to be opted into in the atpkg,
and B) there is no good way to default to the running platform, which is
the sane behavior.

3.  Currently, tools tend to conditionally-compile platform-specific
code.  The effect of this is to complicate bootstrapping, as a Linux
binary *cannot* perform some OSX behavior (and vice versa) because the
necessary code is not present in the executable.

4.  This is not scaleable to cross-compilation (iOS for example is
Coming Soon™ but can't be supported in this architecture)

To work around these problems, we introduce a `Platform` enum, to
replace `PlatformPaths`.  `Platform` is a runtime technology, allowing
for a Linux binary to reason about what the behavior would be on OSX
etc.

Internally, we have three "platforms":

* `hostPlatform`, the platform on which `atbuild` is currently executing
* `targetPlatform`, the platform for which we are compiling. By default
   this is the `hostPlatform`
* `buildPlatform`, the platform where `swift-build-tool` will run.
   This is usually the `hostPlatform`, but if we are bootstrapping it
   is the `targetPlatform` instead.

The user can override the `targetPlatform` by the use of `--platform
foo`.  `linux` and `osx` are supported.  `mac` is supported as an alias
of `osx`.

The primary effect of a platform is to scope tool-specific behavior
(e.g., target=OSX uses the OSX SDK, host=Linux uses a linux path for the
toolchain, etc).

In addition to the tool-specific behavior, we enable overlays for the
target platform:

* `atbuild.platform.linux` on Linux
* `atbuild.platform.osx` and `atbuild.platform.mac` on OSX

This allows packages to reliably perform per-platform configuration in
the overlays.  Critically, some platform overlay is reliably active, so
users in most cases will not have to `--use-overlay` to get proper
platform behavior.

DEPRECATION WARNING: We believe the `swiftc-path` key is no longer
required, as the functionality used can be achieved either by
`--toolchain` or `--platform`.  Therefore, I'm adding a warning to users
that we intend to remove it and to try these features instead.

We need to put out a release with these features (and the warning)
before I'm happy to remove it.  In particular, we use it inside
atbuild/atpkg, and removing it immediately would break bootstrapping, so
let's give it a little time before we tear it out.  We should remove it
from the documentation though.

Per #72, these are not useful

Resolve #72

Thanks to @dunkelstern for his assistance with this.
It required a few minor tweaks.

Toolchain support

Implement packageFramework, a framework packaging tool

Allow variable expansion for shell tool

This allows atbuild to use a different toolchain other than the one we
use to develop atbuild (weekly snapshot).

In particular, this allows you to use "released swift" "xcode swift" or
any other kind of 2.2 Swift.

Documentation PR to follow.

Edited README to discuss atbuild options.

Resolves #58 to my satisfaction.

It was woefully out of date

It is a stupid print and it deserves to die

This allows Mac frameworks to be built directly from atbuild.

This allows the execution of a shell tool with the "collected sources"
of another task.

The intent here is to allow the creation of arbitrary tasks that accept
source files as arguments. xcode-emit is an obvious candidate, and
would allow it to break its dependence on atpkg, which it only uses for
this single feature.

There are other obvious programs that we might want to call with "all
sources" (e.g. preprocessors, etc.) and they may not want to take a
dependency on atpkg either.

See https://github.com/AnarchyTools/atpkg/pull/23