ability to generate warning form state

[wojtask9]

Currently there is no way to generate warning state for form inputs because css rules with state other than valid and invalid is dropped by browsers.

With this PR we are able to generate other states than valid and invalid for example

@include form-validation-state("warning", $warning);

or

@include form-validation-state("info", $info);

Edit: This is useful if we want Server Side validation

[mdo]

This has been shot down at least once before, but suffice to say we're not going to add it as our validation states are based on what the browser provides.

[wojtask9]

@mdo
I understand your concerns. This PR is not about adding or support other states than valid and invalid by default.
This is about utilize existing mixin to generate other states in custom builds.
Currently mixin form-validation-state is large and reusing in custom builds would be great :)