Add customisable language display names

[Nixinova]

The colours of languages can already be customized, and it would be useful to allow the names themselves to be changed as well. For instance I label all my NovaSheets code as Less as that is the most similar language but it's a bit misleading to show that 7% of my code is Less, a language I dont use.

E.g.: plugin_languages_names: "Less:NovaSheets, JavaScript:React"

[lowlighter]

Yeah it shouldn't be hard to implement

[lowlighter]

Shipped in 94539fa 🎉