[refs #935] Add nunjucks templating engine

[zomars]

No description provided.

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[zomars]

I signed it! My company name is: Ancla Studio

[zomars]

This implements what I proposed in #935

[addyosmani]

@zomars Thank you very much for your contribution. We'll apologize for not commenting earlier on the RFC thread you put together. We're currently trying to limit the scope of WSK to as simple a setup as possible for most users. With that in mind, we're not going to introduce a templating system in here just yet.