breakpoint-specific flexbox order

_From @TacticalCanoodle on August 10, 2016 11:3_

I would like to change the grid order with flex order by using classes like grid-item-widths.
For example:

```
<div class="g">
<div class="b0-o01 b2-o02">
<div class="b0-o02 b2-o01">
</div>
```

_Copied from original issue: CastleCSS/castlecss#1_
