Mustaches in attr values are now unwrapped to sexprs during parsing

I cherrypicked one of the technical parts of #23 because that PR may take a few iterations to land.

This commit modifies the HTMLBars parser by unwrapping any mustaches in attribute values to their root sexprs. For example, the template `<button class="btn-large {{dasherize state}} {{other}}"><button>` now stores the class attribute's value as

```javascript
[
  text('btn-large'),
  sexpr(id('dasherize'), id('state')]),
  text(' '),
  sexpr(id('other'))
]
```