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:
> I ran it through callgrind, but I have no idea on how to interprete the
> output of kcachegrind. How can I answer questions such as "Is the
> performance impact neglectable compared to the impact caused by IO?"

Actually it cannot do exactly that, better we look at the data together next 
meeting.