Add universal pos

[proycon]

It might be interesting to map our CGN PoS tags to universal pos tags, a pos tag vocabulary from the universal dependencies project that is in more widespread use (but is much more limited than CGN). This would simply be an extra PoS layer in the FoLiA (perhaps optional).

See:

• https://github.com/proycon/folia/blob/master/setdefinitions/universal-pos.foliaset.ttl
• http://universaldependencies.org/u/pos/index.html