Add tests for issue #1, unqualified references.

+ This demonstrates that an error is generated when a nested value uses an
  extrapolation to refer to a sibling key, without qualifying the key.
+ For example, say we have `{ parent: { x: "{{ y }}", y: "bar" }}`; the
  reference in `parent.x` should be `{{ parent.y }}` instead of `{{ y }}`.