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

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!