Allow training models with non-CGN PoS tags

[proycon]

Frog is currently quite tied to CGN (as we noticed in #52). I propose adding a parameter style for tagger in frog.cfg to indicate whether a PoS tagset is CGN-like (tagger.style = cgn) , i.e. it uses the HEAD(featurevalue,featurevalue) format and to also allow training models that are not in that style (tagger.style = simple?), in which case the resulting pos tags in the FoLiA would of course have no features at all at just be considered blobs. Additionaly, we could perhaps add another parameter value (tagger.style = cgn-full) for a more verbose CGN-style HEAD(subset=featurevalue,subset=featurevalue) (suggested by @JessedeDoes) which would eliminate a lot of the disambiguation problems we currently face.

[kosloot]

Well....
as #52 is solved for now, I see less urgency. I suggest waiting for a real use-case is best.
What probably is only needed, is to make sure that Frog can handle 'unstructured' tags, also without brackets and sub information. It probably already can. (or almost)
A more verbose format looks undesirable to me. More data, more parsing etc.

[proycon]

Yes, agreed