Adding support for highlighting different languages

[beef331]

Presently nimib takes in all code blocks and quotes Henry Ford "You can have all the code you want as long as it's Nim". NbBlock should have a language field to set the rendering inside Md and HTML, allowing nimib to be more flexible for projects like libnimib or nimib.py.

[pietroppeter]

In general I think this is a good idea. Another thing to keep in mind for the planned future refactoring of blocks on which I should write more (or maybe just code).