Improve highlighting of wikilinks and markdown code elements

[martinklepsch]

[[wikilinks]] are commonly used to refer to other vars or namespaces in docstrings. cljdoc supports them (Codox does too) and highlighting those more could be nice. Perhaps the same formatting as regular backtick-code would be just fine.

I also noticed that strings in backticks are only "highlighted" if there's no space in the backtick-wrapped string. Perhaps this is something worth keeping in mind when tackling this issue.