WIP add an Exercise Task supporting multiple formats

Satisfies <https://www.pivotaltracker.com/n/projects/1156756/stories/79368948> but keeping it WIP because I would like to:

- [ ] add code for detecting which item was selected (maybe using [mquan/cortex](https://github.com/mquan/cortex/) )
- [ ] pull the code back into exercise-js

![image](https://cloud.githubusercontent.com/assets/253202/4809360/ec1a9ab0-5eaa-11e4-94b5-07118e5c616c.png)

![image](https://cloud.githubusercontent.com/assets/253202/4809381/113beaf6-5eab-11e4-8f7f-de2aab9473c1.png)
I was originally going to do CSS labeling but decided to create the component to support multioptions like `e) All of (a), (b), and (d)`. But, I might be able to do the multioptions using CSS as well using `:last-of-type`.

Hmm, I might be able to do `.multi-option[data-count]::before { content: attr(data-count); }`