Hosts formatting

This pull request is nearly a complete rewrite of the hosts plugin to support the metadata formatting proposal described in doc/METADATA.ini and discussed in #118. Additionally it introduces a first proposal for a library (keymetaformatting) that encapsulates useful functions to create the comment meta keys.

 Although I could have spent even more time on this topic, I think it is time for a first review.
Actually I am talking about this regex `^[0-9a-zA-Z.:]+$` (the mail view seems to show things a little bit different). But yes we can discuss all this validation related stuff at the next meeting.