Sprint 6 5 footnote plugin

First rough version of footnote plugin.
Can you add the footnote plugin (and CSS) to `build/aloha/build-profile-with-oer.js` ?

You should then be able to build a single `aloha.js` file that contains all the plugins.

The easiest way to run it and test is to do:

```
# Install the `r.js` script that builds a single file 
npm install requirejs

# Build the single aloha.js and aloha.css
./node-modules/.bin/r.js -o build/aloha/build-profile-with-oer.js
```