Add support for Prismic

[aldesantis]

Currently, Renderful is focused on Contentful, but nothing prevents us from creating a generic API that can work with multiple headless CMSs. A good addition would be Prismic.

Mostly, this can be accomplished with a generic "provider" class that acts as a bridge between the API Renderful expects on content entries and the API exposed by the CMSs (Contentful, Prismic).

I'm still not sure how we could handle features that are only offered by one provider but not another. Probably that's just a problem to think about when it presents itself.