Allow to mark code blocks as ascii art in order to allow for better rendering #334

Hi,
I just realized how the ascii art diagrams I like to put into my READMEs really aren't looking very good because of line spacing:

https://github.com/manuelkiessling/soundvenirs-web/blob/09fd7b705d9f479ecbf24710c02d9afd1bd69e08/README.md#the-big-picture

If there was a way to declare the code block that carries the art as "this is meant to be ascii art", and if the Github Markdown renderer would render those blocks with less space between the lines - well, that would be awesome in my opinion.