Rewrite elements with hyphenated tags as blocks

```handlebars
<x-foo src="{{src}}"></x-foo>
```
becomes
```handlebars
{{#x-foo src=src}}{{/x-foo}}
```
---
```handlebars
<x-foo src="{{upcase src}}"></x-foo>
```
becomes
```handlebars
{{#x-foo src=(upcase src)}}{{/x-foo}}
```
---
```handlebars
<x-foo src="a{{b}}c{{d e}}"></x-foo>
```

becomes

```handlebars
{{#x-foo src=(concat "a" b "c" (d e))}}{{/x-foo}}
```
---
**Note:** I modified Handlebars slightly to normalize the isHelper property on sexprs and make testing much less painful. This fix should be pushed upstream, but I'm not sure if we want to be developing against the Handlebars 2.0 alpha yet.