feat: adds syntax highlighting using chroma

[dhth]

Hey 👋

I just came across this repo whilst checking out the original docco project; good stuff.

I see that gocco relies on external commands for syntax highlighting. Sending in a quick PR to achieve the same using chroma. You can see the result running at https://dhth.github.io/gocco/. Using chroma would eliminate the need for an external dependency.

I've also added a Github Actions workflow to deploy to Github Pages (you'll just have to change the source for GH pages to Actions under gocco/settings/pages; lemme know if you don't want that).

Cheers.