Make ./pants help and ./pants help <goal> work properly.

Make ./pants help and ./pants help [goal] work properly.

Fixes https://github.com/pantsbuild/pants/issues/866 and https://github.com/pantsbuild/pants/issues/839

Testing Done:
Options unittests pass. CI baking.

Reviewed at https://rbcommons.com/s/twitter/r/1482/
Sync of https://github.com/pantsbuild/pants/commit/805b980964dcb78c0b6b2d9b8586d5817785650e