Provide something to allow minification and similar processing

[aprescott]

There is currently no specific mechanism in place to, e.g., minify style.css so that it has a smaller filesize.

[aprescott]

This and #5 and #6 might be solveable with the same system of having commands that operate on file extensions (stuff mentioned in those issues). You could define a minification command that operates on files on .css extensions, similar to the command that would hypothetically run on .html or .md files.