[dev] [surf] Add more settings as xproperties

Hi there,

I spent this morning familiarising myself with surf. I love how
simple and sensible the codebase is, making it easy to hack.

I'm attaching 2 patches which make the proxy and custom stylesheet
options changable by xprop. What do people think about the idea of
exposing settings like this? Personally I'm only interested in just
toggling a proxy or stylesheet on or off, which could be easily done
without resorting to xprop. But I wonder what people consider the
best way?

I look forward to hearing people's thoughts.

Nick