Add highlight.js

[Drogglbecher]

For meaningful highlighting we want to use highlight.js. For this it would be good when we update the version via build.rs while building the application. ;)

[Drogglbecher]

One could also think about using npm here and do 'npm update highlight.js' while build process.
So it would be easier to integrate and to stay up to date, on the other hand we would have an additional dependency here.