add code highlight, fixes #9

[mireille-raad]

The current code blocks:

• were overflowing on mobile and causing horizontal page scroll
• there was no code highlighting, code blocks just looked a little different

This PR ads code highlighting...

• for inline code blocks, it still looks like github
• for bigger code blocks, I used a dark theme because it makes code blocks more visible

I used the built-in thing that comes with Jekyll
https://jekyllrb.com/docs/liquid/tags/#code-snippet-highlighting
http://rouge.jneen.net/

For the style sheets, I went with a Pygments theme called monokai

https://jekyllrb.com/docs/liquid/tags/#stylesheets-for-syntax-highlighting
https://jwarby.github.io/jekyll-pygments-themes/languages/ruby.html

Screnshots

mobile is not ideal, the line wrap squishes things

[coolreader18]

LGTM!