Syntax highlighting of files loaded in overlay panes

[PhilippWendler]

In #402, it is suggested to parse the file loaded for link overlays if it is a task-definition file, and provide a richer and interactive view of the file for the user.

We could do something similar for other files as well, for example for C programs we could add syntax highlighting or a list of functions as a table of contents. There exists JavaScript libraries for this so this would not be a huge effort to implement, but we would need to bundle these libraries, so maybe we shouldn't do this if their size is very large.

[PhilippWendler]

Possible libraries:

• google/code-prettify: unmaintained
• highlightjs/highlight.js: seems to be a good choice
• ccampbell/rainbow: unmaintained
• syntaxhighlighter/syntaxhighlighter: unmaintained

So highlight.js seems to be the way to go.