Make Inuit.css unbreakable

This way user can import _vars.scss or use their own config file.
In addition, when pulling down Inuit, you can just drag & drop the
inuit.css and assets folders in and that's it.

If any new variable is introduced, it is going to have a fallback at
_defaults.scss, so all the changes can be non-breaking changes
and _vars.scss can be the variable demo file that it is.