I added a convenience feature, and improved the package file

Please review and pull.

I added a prefixer generator, so one can be all like:

```coffeescript

fs = require 'fs'
Q = require 'q'
prefix = require('prefix-css-node').prefixer '.bootstrap', ['html', 'body']

read = Q.nfbind fs.readFile
write = Q.nfbind fs.writeFile

done = read('my.css', 'utf8').then(prefix).then (css) -> write 'prefixed.css', css

done.fail (err) -> console.error err
  
```