ngram features extractor using spacy

[laugustyniak]

I needed to create scatertext plots with various ngrams length and so it is PR.

[JasonKessler]

Hi Łukasz,

Thanks so much for the PR. It would be great to handle more than bigrams.

A few requests before I can merge this:

• Is it possible to eliminate the cytoolz dependency without incurring a substantial performance hit? I'm trying to keep the number of dependencies minimal.
• Could you please add some inline documentation explaining what the parameters to the various functions are and a small doctest-style example showing how the feature extractor is used.
• Could you please add in some unit tests in the test/ directory. test_FeatsFromSpacyDoc.py could be used as a partial model.

Appreciate your contribution!

Jason