Line number and Handlebars (parser) support

This is certainly dev/experimental material right now, more for code review than PR. Few thoughts:

* Two tests are failing because of a regression in the whitespace collapsing
* There's no runtime support for the new SECTION mustache types
* Not convinced we need SECTION_UNLESS (since it could theoretically be handled by wrapping a SECTION_IF expression in `!( ... )` )