Remove hightlight dependency

[farcaller]

Now that 0.1.3 won't crash if highlight isn't installed it's time to find a better highlighting strategy other than forking to nodejs app.

Pygments can't highlight yaml in any meaningful way, though. See also https://bitbucket.org/birkenfeld/pygments-main/issues/890/yaml-highlight and https://bitbucket.org/birkenfeld/pygments-main/pull-requests/762/improving-lexing-of-yaml-mappings/diff