Include closed part of speech classes

[jmccrae]

Is your feature request related to a problem? Please describe.
This issue discusses the inclusion of closed part-of-speech classes. This includes prepositions, interjections, modal verbs, conjunctions and others.

Describe the solution you'd like
Extend the model to include the words for these part of speech classes.

Describe alternatives you've considered
There is a good question if we need these. They are very hard to define and as they represent only a small number of high-frequency words, it is possible that many NLP applications would prefer to handle these in their own way. I open this issue for discussion