drush help sub-options

Here is a patch that introduces the concept of sub-options for drush help. The advantage of this technique over what was previously done is that commandfiles that extend an option group (e.g. sanitization options, svn options, etc.) can add new items to the group and have them all come out together in the final presentation (instead of all at the end, as they currently are).

This patch only converts one of the existing option groups; if you like the way this works, I'll go back and do them all.