Add box drawing characters to monospace

[otherjoel]

Rec Mono doesn’t work for the box-drawing diagrams I sometimes add in the comments of my source code (example).

A resource that might help with this: https://github.com/adobe-type-tools/box-drawing

[arrowtype]

Thanks, @otherjoel! I'll probably add this to the Recursive Code project (https://github.com/arrowtype/recursive-code-config). It's fine to leave the issue here, though.