Ability to configure Mincer dynamically added (`configure: (mincer) -> ...`)

Required i.e. to add support of `nib` to Stylus. Example:

```coffee
    mince:
      styles:
        include: ['stylesheets', 'public']
        src: 'application.styl'
        dest: 'public/assets/application.css'
        configure: (mincer) ->
          mincer.StylusEngine.registerConfigurator (stylus) ->
            stylus.use require('nib')()
```