Replace mustache.js with own fork

[neocotic]

We've always used a custom implementation of mustache.js, however, I'd like to move our modified version into a separate repository to make maintaining it much easier.

https://github.com/tmplat-extension/tmplat-mustache

We should then depend on it within the extension.

[neocotic]

Alternatively, I'm considering about simply replacing our custom implementation with pure mustache.js, however, this would be a massive breaking change and I doubt users would understand or be happy with it, so I'm simply considering it at this stage. It would make maintenance much easier.

That said; using the latest version of mustache.js is proving difficult. Mainly because it's much better structured than 0.7.2, which our custom implementation is currently based on. I'm tempted to just fork this version and use our file as-is.