Make !important configurable.

[adamjohnson]

Right now, !important is applied to a lot of rules--likely intentionally.

Sometimes, users might not want to add !important to rules in their project. It'd be nice to be able to turn this on/of globally, or on a per mixin basis.

Code example:

$global-important: !important !default;
@mixin myMixin($is-important: $global-important) {
  color: red $is-important;
}

.example {
  @include myMixin();
  font-size: 16px $global-important;
}

Outputs:

.example {
  color: red !important;
  font-size: 16px !important;
}

Users will be able to explicitly override !important on a per mixin basis by doing:

.example-2 {
  @include myMixin($is-important: null); // `null` is used to omit `!important`
}