Add context stack and related tests from the spec

[briandunn]

I needed this to enable basic conditionals. I feel {{#feeling}}much {{feeling}} for {{/feeling}}you.

Tried to only ad formatting updates on lines that I had to change.

[schultyy]

Hey @briandunn!
Thank you so much for submitting this PR. I know it's been a while since you opened this PR. I see there's a few merge conflicts. If you don't mind, could you take a look at these?