automatically populate footer with git variables

Attempt at a zero-config fix to #52.

The footer contains a link to the github repository and an email
address. These will vary by lesson. To avoid configuration,
automatically determine what these should be by looking at the output
from `git config user.email` and inspecting `git remote -v`.
Yeah it's pretty hacky. A config file would be cleaner but you could use this to auto generate config in the case that it isn't there. Might be a bit too magical though.