Add supporting evidence

[brynrhodes]

The ability to specify and capture supporting evidence for each population as part of a quality measure is being added to the quality measure IG. Examples of this are available here:

HL7 JIRA Tracker: https://jira.hl7.org/browse/FHIR-53432

Extensions PR that creates the extensions:
HL7/fhir-extensions#224

This information should be reflected in the measure liquid templates