Add spacy_install_langmodel()

[kbenoit]

Following this comment on the wishlist thread by @mshariful:

We should add spacy_install_langmodel() that would take a local language model file and "install" it in the same way that spacy_download_langmodel() does from the spaCy remote repository. This could then be used in spacy_initialize() in the same way that downloaded language models are usable.

[kbenoit]

This is a really good idea that would offer a solution for #180.