Added support for highlight.js

[xgvargas]

Hi, First of all I have to say I love to use Jingo for documenting our work. But in my current project I use lots of YAML and the 5 years old node-syntaxhighlighter is not able to deal with it. Acctually it can handle a very small number of languages, so I've made this PR wich adds support for hightligh.js for jingo with more then 127 languages and many styles (but current code has a hard coded style, I can add a config property for this IF you this this PR is worth).

[claudioc]

Hi @xgvargas

good idea! Together with its markdown parser, the syntax highlighter is definitely something Jingo needs to improve upon. I'd totally consider merging this PR: I just need to take a look at the changes I need to do at CSS level (as you mentioned) and think about any "breaking changes" we might introduce (you know, the problem when changing something "low level" is to guarantee not to break how people are already using Jingo).

Changing the syntax highlighter should be much easier than changing the markdown intepreter, though.

I will keep you posted!