Default to former TextCategorizer model
Default to former TextCategorizer model

* Keep TextCategorizer default model same as v2.0
* Add option 'architecture' that allows "simple_cnn" to switch to
simpler model.
* Add option exclusive_classes, defaulting to False. If set to True,
the model treats classes as mutually exclusive, i.e. only one class can
be true per instance.

explosion/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,explosion/spaCy,explosion/spaCy,spacy-io/spaCy,honnibal/spaCy,honnibal/spaCy,honnibal/spaCy,spacy-io/spaCy,honnibal/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy