Renaming the tag with an underscore lets us add it to the tag map
without worrying that we'll change the sequence of tags, which throws
off the tag-to-ID mapping. For instance, if we inserted a 'SP' tag,
the "VERB" tag is pushed to a different class ID, and the model is all
messed up.
* Move v2 parser into nn_parser.pyx
* New TokenVectorEncoder class in pipeline.pyx
* New spacy/_ml.py module
Currently the two parsers live side-by-side, until we figure out how to
organize them.