Adds nunjucksify.extensions to configure template file extension.

I personally dislike the `.nunj` extension and prefer to simply use `.html` for the template files. When using browserify's JS API (for example in a Gulp task), you can now configure the extension with:
```nunjucksify.extensions = ['.html']```

Added test, but no docs, since it was not clear where to add it, and not sure how you would configure this in another setup other than Gulp.

Salute!