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:
> In fact it is already "Comment keys that are not above any key (comments in
> the last lines of files) are added to the parentkey." ;)

Yeah, I checked in my doc/METADATA.ini and not the one of the merge request ;)