Syntax highlighting

Highlight the implementation code.

- [x] Implement syntax highlighting using `minted` *(in b43b38d)*
- [x] Implement option to use syntax highlighting *(in 7e51b71)*
- [x] Implement fallback (back to standard verbatim) if no `pygmentize` or no `-shell-escape` *(in 0a5a946)*
- [x] Fallback if `minted` is unavailable? *(in abeffe0)*
- [x] Implement a custom pygments theme that fits with the overall theme *(in 193df17, 89ecd0b)*
- [x] Fix spacing issues *(in 2f38765)*
- [ ] Is it possible to make `pygmentize` recognize `@`/`_`/`:` as parts of a command? *(looks like this isn't possible without modifying the lexer)*
- [x] Clean up the code, make it LaTeX3-y *(code is clean and LaTeX3-y enough with the exception of the Pygments-generated parts, and I won't reimplement those as LaTeX3)*
- [x] Document the feature completely *(in 7e51b71)*
- [x] Synchronize font settings of highlighted and verbatim code *(in 2f38765)*