Interpolate fact values

Attempts to retrieve a fact value for the given slug when the expected
fact marker is encountered. Replaces the marker with an empty string if
no value is found.
Update search endpoint to work with new api adapters.

They've been updated so that the rummager client inherits from GdsApi::Base, and uses the common exception classes.
Update fact regex to be case insensitive.

Use GdsApi::Helpers to construct FactCave instance.
Also add integration tests, and improve unit tests.
Bump api adapters to 7.2.0 to pull in fact_cave client
Add comment explaining regex
Merge pull request #107 from alphagov/interpolate_fact_values

Pull in fact values in markdown fields.