Made at2x require a hi-res tag in the value to activate

As Discussed in [Styl #21](https://github.com/visionmedia/styl/issues/21), I updated the `at2x` plugin to require `hi-res` in the value to activate. This lets authors be more selective on its use, and makes sure that people are consciously accepting the consequences of using the plugin on specific images.

Please let me know if you want anything else tweaked.