Support for mixins

[tomasdev]

In the SPEC.md and in the website, shows that there should be support for mixins, although that is not yet implemented into the code.

This issue is to track progress on that. Everyone is invited to do a pull-request of it.

Mixins should require a Section but not markup, instead, usage.

[jsmestad]

Would love support for this as well. I did find a branch that did support SASS/LESS mixins here