feature: support [hash:\d] and [chunkhash:\d] filename templates

Thanks for an awesome plugin! This PR adds support for ```[hash]``` and ```[chunkhash]``` filename templates so you can have ```filename: '[name].[chunkhash].js' type filenames and get out a CSS file named something like ```app.12345678.css```. Obviously the chunk hash is the same both for the CSS file and the file it was split out of.