Support @root

[bobbyshaw]

I'm not sure whether this is a handlebars version issue or not but the latest version supports an alternative way of defining a context path: @root.

It would be nice to have access to this.

For context, we were having an issue with path expressions. Our particular edge case is that the path required was different in page builder preview for a new widget then it was once published. Having access to @root would perhaps have given us another route to a solution.