Add rig.simple, for breaks on single element

Fixes #1
My preference would be to omit default values and encourage usage to be very intentional, especially the breakpoints which are arbitrary.

For common use cases I'd rather pass these as params inside of a dedicated method. Something like:

```coffeescript
Rig.resize = (block, config, selector, breakpoints, sizes) ->
  return Rig.simple block, config, selector, 'passthrough', {}, breakpoints, sizes
```