Global mixins (for pug templates)

[akyoto]

One should be able to write global mixins which can be called from any Pug template.
A classic example is to have a global mixin for rendering avatars / gravatars and simply calling +avatar(user) in the templates which keeps everything DRY and simple.

The basic concept is to add a new folder mixins where each .pug file will be loaded and included for usage in other page templates.