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/
Kill pants own use of the deprecated goal command.

This pass does not go for docs, just code and scripts.

Testing Done:
Locally:
```console
$ ./build-support/bin/publish_docs.sh -o
$ ./build-support/bin/release.sh -n
```

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/43877791

Bugs closed: 864

Reviewed at https://rbcommons.com/s/twitter/r/1479/
Fix missing import.

Reported by Zero Cho.

Reviewed at https://rbcommons.com/s/twitter/r/1483/