Fix incompatibility between CSS extras & the WPD plugin

Fixes issue #144

Just for reference:
> The wpd-plugin assumes, that `Prism.languages.css.selector` is [just a regex](https://github.com/LeaVerou/prism/blob/gh-pages/plugins/wpd/prism-wpd.js#L11), but if you load `css-extras` it is not just a [regex anymore](https://github.com/LeaVerou/prism/blob/gh-pages/components/prism-css-extras.js#L1).