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.
Put delegated methods back

(this commit will totally disappear in rebase
but I want tests to stop failing)
This line should have been in the big country
commit, going to be less painful to just add it again in
rebase.
Fix for text answer interpolation.
WIP commit, failing spec for interpolation of
country answers.