Add instructions about the use of conditionals
Improve error message on undefined value
Add interpolator
Rename NodePresenter to ConditionalResolver
Refactor node presenter to invoke conditional resolver
Integrate variable interpolation to Engine#evaluate_node
Add interpolation instructions to readme
Bugfix: interpolation needs to work with parslet slices

the parser actually produces parslet 'slices' rather than strings.
A parslet slice behaves like a string but doesn't have all of
the methods of string.

So, interpolate needs to #to_s the thing it receives to be able to
handle slices.

This test is to document that fact and catch any regressions.
Merge pull request #13 from alphagov/interpolation

Interpolation