Generalize input groups about mini.css

[Chalarangelo]

This idea sounds interesting, however it might take up too much code, which will bloat the module. I might try to create something like what you suggest but with a smaller scope. This addition might be a long way down the road, as there are more pressing things to deal with (new flavors and some components that have been discussed more than this one).

[Chalarangelo]

Implementation of this feature has begun. In fact, the input-group component will be extended significantly with something like a .responsive class to allow responsive input groups to be created. The old-school input-groups will not be removed, however, as they are essential to the checkbox and radio components.