Pull exercise footer instruction out to .md file

- Makes it easier for workshoppers to customise the instructions
- e.g. `run` command doen't work for `nodebot-workshop` and we'd like to add a line to say "do a `node program.js` to try your solution out for real.
Fix typo in footer.md
Provide footerFile opt when creating a workshopper
Merge pull request #57 from tableflip/master

Pull exercise footer instructions out to .md file