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.
you wrote:
> > +                             KEY_VALUE, "/*",
> But what is the purpose of this exact occurrence of check/regex?

This regex is from glob and has nothing to do with regex validation.

The validation framework is better discussed in the next meeting ;)