report: add syntax highlighting

[piotr-binkowski]

This fixes #162 and adds syntax highlighting using pygments.
Syntax highlighted sources look like this:

[mithro]

LGTM.

[hzeller]

Very neat.

Can we modify the output a little with the configuration of pygments ?

• on the top, there is some whitespace. The generated code seems to create some empty<h2></h2> for instance
• If all line-numbers could contain an <a name="L42">42</a>, so that we can directly link from a logfile path/to/test/file.sv:42