Support -h

[Mariatta]

Closes #88

$ python -m cherry_picker -h
Usage: cherry_picker.py [OPTIONS] [COMMIT_SHA1] [BRANCHES]...

Options:
  --dry-run           Prints out the commands, but not executed.
  --push REMOTE       git remote to use for PR branches
  --abort             Abort current cherry-pick and clean up branch
  --continue          Continue cherry-pick, push, and clean up branch
  --status            Get the status of cherry-pick
  --push / --no-push  Changes won't be pushed to remote
  -h, --help          Show this message and exit.