Update for recent Handlebars features/syntax

This gets the plugin's parser up to date with the latest features/syntax in Handlebars.

Specifically, the following changes were made:
* `{{@data}}` syntax is no longer marked as an error (and the highlighting of these data vars is configurable)
* Escaped mustaches are now properly highlighted, so `\{{escaped}}` constructs do not render as regular mustaches
* Single quotes in mustache strings are no longer marked as errors.  i.e. `{{foo bar='baz'}}` behaves as expected
* Handlebars block comments (`{{!-- I am a block comment which can contain {{mustaches}} --}}`) are now supported
* Partials names can now be paths: `{{>path/to/partial}}`
* Negative number literals no longer marked as errors: `{{foo bar=-2}}`