Ability to load configuration from purgecss.config.js

[akrawchyk]

I'd like to provide defaults for purgecss-webpack-plugin and allow the users of my plugin to provide their own whitelist configuration. The docs indicate we could add these options in the purgecss.config.js file: https://www.purgecss.com/configuration#configuration-file

But, purgecss doesn't load configuration from this file if its passed an object: https://github.com/FullHuman/purgecss/blob/master/src/index.js#L41-L42

And purgecss-webpack-plugin passes an object to purgecss: https://github.com/FullHuman/purgecss-webpack-plugin/blob/master/src/index.js#L118

So that ignores the ability to load configuration from a file.

Maybe this is an issue of wanting my cake and eating it too, but it would be nice to be able to configure these options separate from the webpack configuration.