Enable Web workers

[niklaszantner]

It would be nice to have the possibility to enable the usage of Web Workers with the directive (as described here: https://highlightjs.org/usage/). Maybe some attribute like use-web-workers which could be set to true.

[JayChase]

I like the idea of this. I will start having a think about how to do it in an Angular friendly way for the next version (it might be a while though).