Add program listing example

As reported by jo3w4rd on #65 on github.
Improve handling when project is not found in config
Provide support for programlisting

We have to parse the programlisting tags within para tags and then
provide renderers for programlisting, highlight and codeline.

We render as a line block instead of literal_block as there can be
references to other parts of the code base which the literal_block
doesn't seem to handle well.
Document custom css usage
Switch to literal block for programing listing

Turns out that if we pass an extra empty string to the literal_block
node at the start then it renders the pending_xref properly. This makes
no sense to me but hey, I've never really grokked this stuff anyway.

Consequently we can remove the CSS stuff, which is good.
Acknowledge jo3w4rd in the README and credits

Thanks for reporting the issue with programlistings.
Merge branch 'programinglisting'