Enable handlebars ast

This PR aims at providing a hook to provide handlebars AST directly. I named it `Handlebars.ast_to_json_ast`, hopefully that makes sense.

This is kind of required for any package that wants to generate Handlebar ast, for example my [`emblem` package](https://github.com/marcandre/meteor-emblem) package. 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).