Add template & compile functions for HTMLBars.

This provides a central location for any customizations we need to do to the compiled templates from htmlbars-compiler.

Things like `isTop` and `isMethod` being set for example. These are obviously, Ember concerns (neither thing really makes sense in HTMLBars).

The node-land precompiler should now return the following template:

```javascript
export default Ember.HTMLBars.template(/* output from compileSpec */)
```