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 am wondering if we could somehow create "HTMLWithPlaceHolders" component so that the the placeholders can be react components.

```
<HTMLWithPlaceholders innerHtml={ stem }>
   <BlankQuestion answered={!!config.answer} store={this._answerStore}></BlankQuestion>
</HTMLWithPlaceholders>
```