Add rig.simple, for breaks on single element

Fixes #1
or rather, something like:

```coffeescript
Rig.resize = (block, config, options) ->
  return Rig.simple block, config, 'passthrough', {}, options

Rig.resize block, config,
  selector: '#media'
  breakpoints: [504, 1008]
  widths: [200, 400, 800]
```