added custom loaders to nunjucks-template-engine

[WoMayr]

PR for Issue #224.

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[WoMayr]

I signed it!

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[petebacondarwin]

@WoMayr - thanks for the PR. My only concern is that this does not give an option to remove the default loader if one wanted to. I wonder if we could find a way to include the default in the loaders property? Given that the default loader needs access to the templateFinder.templateFolders for it to work, it might need to accept an array of factory functions?

[WoMayr]

This also came to my mind. But I didn't have any good idea on how to fix that.

Maybe allow to either pass a factory method or directly the instance of the loader and use this to add the loaders like it is done with the filters and tags inside /nunjucks/index.js.
But I have no idea how the dependency injection works to call the factory method with the wanted parameters.

[petebacondarwin]

I don't think this PR as it stands is the approach we want to take.