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:
> Even if I just moved it? split would use a kdbproposal function then

What do you mean?

ksDeepDup should not be in kdb.h because it was never there before. If you 
want to propose it as API, you can add it in kdbproposal.h.

split* are internal functions, they never will be in kdb.h.