Allow customising the code font

[rombert]

The code font seems to be harcdoded to Nimbus Mono L

• https://github.com/thorstenb/odpdown/blob/6e6ff4ed91b13a64862bb0476a1ae4f5cbc1845c/odpdown.py#L437-L443
• https://github.com/thorstenb/odpdown/blob/6e6ff4ed91b13a64862bb0476a1ae4f5cbc1845c/odpdown.py#L461-L462
• https://github.com/thorstenb/odpdown/blob/6e6ff4ed91b13a64862bb0476a1ae4f5cbc1845c/odpdown.py#L472-L478

While it is a nice font, I would prefer being able to change it. Also I'm not sure whether this font is always installed