1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

Previously, we assumed the first build configuration would have one. Now we
keep looking until we find one. Also, re-ordered the configurations so that
you'll get the Android one if you have both, which is probably what you would
expect.

Fixes #100

This flag builds a local FLX file and pushes that to the device instead of
using an HTTP server.

Instead of calling through `pub` to invoke build, this patch refactors the
build command so that it can be called directly.