RFC: Update generate-at-breakpoints mixin

Allow a mix of min and max media queries. eg 
```
@include generate-at-breakpoints('.u-position-fixed', (400 max, 401 min, desk max)) {
  position: fixed;
}
```

Change max-width media queries generated by the mixin from this format `.u-position-fixed-at-400` to `.u-position-fixed-up-to-400`