Allow using css-minify instead of lighteningcss

[ctron]

While I see the benefits of lighteningcss, there are some downsides because lighteningcss never releassed a stable version, only pre-releases. That might cause a major headache when an newer version is incompatible with the API.

At least, minify-html should use a pinned version, like =1.0.0-alpha.51.