Support a backend for standard git servers

**Is your feature request related to a problem? Please describe.**

I like the Netlify CMS UI for editing content, but afaik the only option for self-hosted repos is to set up local instances of either GitLab or Bitbucket, both of which are rather heavy weight for low-end hardware.

**Describe the solution you'd like**

I'd like to see a straight `git` backend that stores content on a standard git server. I can think of two ways to implement this:

- a gateway that accepts requests from the backend and communicates with the git server
- a backend that accesses `git-http-backend` directly with something like [js-git](https://github.com/creationix/js-git) or [isomorphic-git](https://github.com/isomorphic-git/isomorphic-git) (CORS headers required for cross-origin repo access)

Have a missed some way to do this already, or am I over-simplifying this request in some way? :)

**Describe alternatives you've considered**

**Additional context**