Feature: GetDefinition(paramOrOutput) function(s)

[vdice]

We've seen the use for helper func in cnab-go, GetDefinition(paramOrOutput) or some such (may need a function for each type), that'd return the schema definition for a parameter/output or return a corresponding error (if definition not found/empty/etc.) Duplicated/boilerplate code for retrieving a parameter's/output's definition can then be reduced to a single function call and the implementation updated in one place as needed, etc.