Loader support for nunjucks

I am using consolidate.js for my framework [squareboy](https://github.com/harsha-mudi/squareboy).
I use  nunjucks with template inheritance, for my site. 

This code, allows the user to pass a custom [loader](https://mozilla.github.io/nunjucks/api.html#filesystemloader) in the options hash.

If I am not mistaken, swig also needs something like this.