Add breakpoint value to settings maps, add simple-susy-breakpoint mixin

[jdsteinbach]

Here are my thoughts behind this: I'd like to attach breakpoint MQ-data directly to a settings map (yep, that means a tighter integration with Breakpoint). I'd also like to shorten the syntax for generating new grid styles in a new breakpoint context. Rather than:

.selector {
  @include susy-breakpoint($mq-data, $small-settings) {
    @include span($span-stuff);
  }
}

I'd prefer:

.selector {
  @include simple-susy-breakpoint($config, $columns, $grid);
}

In the request I've submitted, the $config settings map will have a breakpoint key that would hold a value/list that's compatible with Breakpoint. The mixin would get that value automatically (I use a settings map per breakpoint if the column count or proportions change responsively), then get either a span value (span, location, layout) or the word container and call span() or container() from $columns. If it's a container, the $grid parameter would tell the mixin to re-run show-grids() at this breakpoint.

Not sold on the name at all - please suggest a better one (or we could work to add this functionality to susy-breakpoint() while maintaining backwards-compatibility.

Thanks!

[mirisuzanne]

I guess this has been sitting around for a while. I think it's covered by the existing susy-media and susy-breakpoint that we added a while back? Let me know if I'm wrong.

https://github.com/oddbird/susy/blob/master/sass/susy/language/susy/_breakpoint-plugin.scss