Basic support for the "with" template helper

[creature]

One of Handlebars' built-in helpers is {{with}}, which allows you to change "contexts" in a template. Basically, it makes it easier to use nested objects. As the Handlebars documentation demonstrates, given this data:

{ person: { firstname: "Yehuda", lastname: "Katz" } }

Then you can write this template to render "Yehuda Katz":

{{#with person}}
{{firstname}} {{lastname}}
{{/with}}

This PR adds basic support for this helper, along with some test cases (taken from the Handlebars documentation) to demonstrate it works.

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One wrinkle with this PR: it doesn't support relative paths, as that's not yet supported in ruby-handlebars. I've included a (skipped) test case that demonstrates this (which works in the Handlebars playground), but I can't figure out how to add parser support for that. Some help here would be very welcome! I'd also like to add support for {{lookup}}, but getting relative paths to work is a prerequisite for that.