Introduce data modules to Node Transformer

Node interpreter now takes an optional data module,
this is similar to existing plugins, except that
they will be available at Flow build time, opposed
to render time.

Monkey patching parslet was the least painful way
to do this, otherwise we would need to introduce a
notion of partially created Elements and have a
post-transformation step.
Interpolation for country answers

Also fix for text answer interpolation, and tests
for multiple choice answer interpolation.
Add description of country questions to Readme.