Issue 18 add help flag

Add a --help flag to the command line.

This prevents the error `hi: unrecognized option '--help'`.

This is for issue https://github.com/fujimura/hi/issues/18.  It is based off of the code for pull request https://github.com/fujimura/hi/pull/20 so pull request https://github.com/fujimura/hi/pull/20 should be merged before this is merged.