Add support for [hash] in extract

[ba55ie]

To safely use long-term caching I would like to be able to add a hash to the file name. Just as you can do with the entry or chunk file names in Rollup.

extract: 'dist/my-custom-file-name.[hash].css'

Or an output option like rollup-plugin-scss which enables you to add your own hash (e.g. using rev-hash).

[ba55ie]

With extract: true and a configurable output folder as mentioned in #284 this will be resolved.