add an element generator, similar to component generator

[codingdesigner]

I think we need an element generator that works nearly the same as the component generator, except that it creates the sass for _item _extend and _mixin in the elements directory, and doesn't create html templates or components.

[Snugug]

Also refactor elements so that they can be self-documenting the same way components are (or do by hand)

[Snugug]

As there is no longer a need to wrap the extends, do you think this is still a priority?