Allow required unmatched positional arguments

Hello!

I have really been enjoying using nomnom, but I have found a use case I was unable to perform.  I wanted to allow the user to specify a command, and have a trailing list of unmatched positional arguments, but at least one was required.

The problem therein was that, it is not possible to specify positional arguments that behave this way, as I do not know before hand the index at which they will begin.

I am not sure if this is something you want to pull in, but I think others could find it useful, e.g. as a workaround for https://github.com/harthur/nomnom/issues/12 .  I tried to follow your code conventions, but please let me know if this pull request needs any work - if you decide to pull it in I can document this feature in the README.

Thanks!