Module grouping into folders

[MarcDiethelm]

This is very helpful for large projects with tons of modules.

Simple implementation: Allow grouping of modules and just generate one set of assets as before.
Advanced implementation: Create a set of assets (bundle) for each group. Make it available as a template var.
Extra nice: Generate a sprite file per group, once spriting from modules is possible.

Slightly OT but... Asset grouping outside of modules (inline, base...) would be a huge problem to implement. https://gist.github.com/MarcDiethelm/7e5fc9c053546f1501cf