Support completions of subcommands

With this change gemset subcommands will also support completion of their arguments.

What it does is simply passing the --complete argument on to the subcommand if the subcommand contains the comment ```# Provide rbenv completions``` like rbenv-completions does.

There is one small snag with this change in that it wont work without this: https://github.com/sstephenson/rbenv/pull/267

You can test it in action with a command like ```rbenv completions gemset create```