Doesn't work with assets generated by Jekyll-Assets

[philnash]

When working on philna.sh I noticed that this successfully compresses HTML files, but the assets are generated by jekyll-assets (currently version 2.4, though this should be upgraded to version 3) and are not compressed.

Need to look at how Jekyll Assets handles the files and if this project can hook into the Jekyll Assets process to generate compressed files too.

[philnash]

Perhaps this shouldn't be the job of this gem. Assets that are looked after by an alternative gem should be compressed by that gem, or an extension to that gem, themselves.

Perhaps look into including gzip/zopfli/brotli compression in jekyll-assets first.

[philnash]

This now listens for the Jekyll::Assets hook and compresses everything in the output directory.