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/