Add support for responsive utilities

[hugooliveirad]

Would be great to have an optional set of responsive utilities, like Inuticss' responsive spacing utilities.

[johno]

Yeah, I'm collecting some ideas for "extras" that people could optionally include, like so:

@import 'furtive/scss/all';
@import 'furtive/scss/extras/all';

These responsive utils should definitely be included in the extras. I think it will also be useful to use these mixins to create the current spacing elements within Furtive, too. That way adding additional classes for spacing is as simple as adding some additional SCSS variables.

This is great, thanks!