Calling full model name by default

[amatsuo]

At the moment, spacy_initialize use the default model name en. This would become a bit confusing as we are planning to use word-vectors that come with spaCy language models and vectors come only with _lg or _md (#171). Once this is implemented, we expect a number of spacyr users will have multiple models from the same language.

I think we should use a full model name as the default (e.g. the default will be en_core_web_sm) to avoid this confusion.

That's also what @ines recommends:
https://support.prodi.gy/t/spacy-load-en-download/1858

This will be implemented with #171 (unless vetoed by @kbenoit or someone else).

[kbenoit]

I agree, let's implement this in the next update. But as a transitional step, we should map the two-character language names to the "default" language models, if those exist, and print a warning.

[kbenoit]

Now in master.