[Proposal] Replace html-minifier with htmlnano

[`html-minifier`](https://github.com/kangax/html-minifier) is not actively maintained anymore. Although there is a forked version [`html-minifier-terser`](https://github.com/terser/html-minifier-terser), it only replaces `UglifyJS` with `terser` to support ES6 (inline `` affected only).

There is another HTML minify tool [`htmlnano`](https://github.com/posthtml/htmlnano). I have already used it in my project (and my personal blog as well). It doesn't receive active maintenance for a while, but now I am porting `html-minifier`'s features to `htmlnano` (https://github.com/posthtml/htmlnano/pull/94, https://github.com/posthtml/htmlnano/pull/98, and more to come).

Also, `htmlnano`'s source code is clean and its maintainer @maltsev is currently active, which means it might receive more community contributions in the future.