Check for missing variables that are specified in headers, body or auth

[Alexandre H.T.R. Bonfitto]

Based on Postman issue:

postmanlabs/postman-app-support#285

If a request header or body, or auth parameter, uses a variable template and there are no effective variables that can supply the value, a validation error should appear.

This behaviour should be optional, because sometimes the variable template should be as is.