Add support for switching code block language

[mzanetti]

In order to simplify some docs pages I'd need to be able to define code blocks in multiple languages with a tabbar to switch between language...

Now, not all code blocks will support the same set of languages... i.e. some might only be supported in C++, others will only list JSON or bash, and some would allow switching between C++/JS/Python.
I guess the best would be if code blocks within one .md file would be synced up so that the user doesn't need to switch each block individually, yet this shouldn't break if one of the blocks doesn't support the same set of languages