Feature: Detect missing init.js data fields used in templates

[ryan-roemer]

Scenario: We have a templates in <archetype>/init/ that have some data field that is not gathered / created in <archetype>/init.js

Task:

• Add some way of inferring all needed props in templates and erroring if not present
• Complexity: But, need to handle props that can be empty... (shouldn't be an issue?)