[add] explicit syntax highlighting language override

[lennart]

Hi Jerome,

I just experienced some issues with highlight.js' language auto-detection and decided to use marked's ability to parse

```any-given-language # => is parsed as 'any-given-language' in token.lang 
random:= code;

for explicitly highlighting a certain programming language.

This pull request includes a simple guard to only use this feature if `any-given-language` is containd in `highlight.js`' loaded syntaxes (`hljs.LANGUAGES`)

[jeromegn]

Sound about right!