Highlighting codeblocks in README's and CHANGELOG's.

[MangoIV]

CommonMark already does the right thing when converting a codeblock of some language lang to html; it wraps it in a <code class="language-lang"> tag. Incidentally, this is exactly what a javascript syntax highlighting library like prism.js (https://prismjs.com/#basic-usage) wants to see when it is used for highlighting codeblocks. I think this would quite a nice low-hanging fruit to add to flora which would really make the landing pages of all kinda package more enjoyable.