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`.
To change this to use a `_config.yml` instead, replace this with `pandoc _config.yml --template _includes/footer.html`.