Extract compressor as generic gem

[dhh]

Tailwind CSS needs purging, but it's a generic technique that could be used with any css toolkit. Should extract this into a generic gem. Maybe sprockets-css-purger, then this gem can use that, and focus mostly just on configuration and shipping the tailwind css.

[dhh]

This is btw and open invitation to anyone who'd like to work on this 😄

[dixpac]

@dhh I will give it a go, after I finish the scaffolds.