Nunjucks: Allow adding custom loaders

[WoMayr]

I am currently missing an option to add custom loaders to the nunjucks templating engine.

In my opinion this would be easy to fix by adding loaders similar to filters to nunjucks-template-engine.js

[petebacondarwin]

A simple workaround would be to provide your own implementation of templateEngine which adds these loaders.
But happy to accept a PR that updates the standard one with an option to add loaders.

[WoMayr]

I sent you a pull request