Add basic setup command

To make Flat installation quick and painless, there is now a setup command.

When Airship is run with this command (e.g. ./airship setup) it will install Airship and all the necessary Flat dependencies from their sources.

Add --target-runtime CLI arg

You can now use the --target-runtime CLI argument to target specific runtimes for runtime-specific code. For example, --target-runtime browser can be used to only compile browser-specific code. An example use case of this is if you want to handle logging differently depending on the runtime. In node, you may want to use process.stdout instead of console.log for messages:

[TargetRuntime browser]
logMessage(String message) {
  external js {
    console.log(#{message.chars.data});
  }
}

[TargetRuntime not browser]
logMessage(String message) {
  external js {
    process.stdout.write(#{message.chars.data} + "\n");
  }
}

General bug fixes

Fix ./airship clean command.

Rebuild dependency after updating dependency.