Expose icons as variables

[sveetch]

This will be usefull to directly include icons in element rules without use @extend.

Dont know exactly how to expose them so they can be used project settings without more complex installation procedure.

Once done, a mixin could be added to create every properties needed to add icon (not only the icon character, but also font-family and stuff). But this may not fit in django-icomoon because it's sass stuff.

[sveetch]

Actually it can be achieved just by changing icomoon/icon_map.css, maybe add another template to build some optional SASS will be enough.