Add a new template engine

[jwir3]

This project works great for mustache. I've found there are some issues using handlebars, but these appear to have already been reported.

One thing that would be really nice is if we could add another template engine. Specifically, for our work, I'd like to add nunjucks. This, of course, is hindered by the fact that there doesn't seem to be a nunjucks-parsing gem available for rails.

Does anyone know how much of an effort it would be to add the aforementioned gem as a wrapper to the existing nunjucks parsing, then use it within stache?