Pull exercise footer instructions out to .md file

Makes it easier for workshoppers to customise the instructions.
e.g. `run` command doesn't work for `nodebot-workshop` yet, and we'd like to add a line to say "do a `node program.js` with your arduino plugged in to try your solution out for real."

We lose the if help file exists smarts, but I think it's more useful to have the option to customise it. We could probably get it back with some handlebars style `{{#if help}}` magic.