Add root context lookup

Access to root context in partial by `{{root}}`.

ex.
```
---
name: foo
bar:
  name: bar
---
{{name}}
{{> leaf bar}}


leaf.hbs:
{{name}} {{root.name}}
```
of
```
foo
bar foo
```


See https://github.com/wycats/handlebars.js/pull/525#issuecomment-25759857