Documentation spacy-stanfordnlp, custom tagset map

The documentation has been greatly expanded. The most important addition to the README is the mention and explanation of using spacy-stanfordnlp. spacy_conll can be used together with this spaCy wrapper around stanfordnlp. The benefit is that we can use Stanford models, with a spaCy interface. From a user perspective, this means better models, guaranteed Universal Dependencies tagsets, and an easy API through spaCy. (The cost is that Stanford NLP models are significantly slower than spaCy's models.) Small tests for spacy_stanfordnlp have been added.

A new feature is that you can now add a custom tagset map (conversion_maps). The idea is that you, as a user, have more control over the output tags. You can for instance specify that all deprel tags nsubj should be renamed to subj. This is useful if your model uses a different tagset than you want. See the advanced example in the README for more information.

This release closes:

• "The dependency relations aren't transformed to universal dependencies" (#4)