Allow custom @content to be passed into classes mixin

[djgrant]

Allow the custom @content to be passed into the classes mixin:

@include classes('my-selector', $on: 'tablet') {
  padding: 20px;
}

If a placeholder exists with the same selector name, both the styles of @content and %placeholder will be used.

(It's not possible to test if @content is passed so whether it's ideal or not this is the behaviour we're getting).

[djgrant]

Basic implementation:

@mixin classes($selector) {
  .#{$selector} {
    @content;
    @extend %#{$selector} !optional;
  }
}

Currently %#{$selector} is not optional. That behaviour is quite handy for cases where catching instances where the placeholder has not been defined. Should considered if removing that is really a good idea.