Enable handlebars ast

This PR aims at providing a hook to provide handlebars AST directly. Maybe it should start with an `_`?

I kind of need it in my [`emblem` package](https://github.com/marcandre/meteor-emblem) package, since Emblem is a nice library that translates text into handlebars AST.

This PR depends on #1790 (since `emblem` requires handlebars 1.0.10+ because of the new AST).