Mechanism for helpers to specify valid app.yml usage

[macMikey]

I'm just picking on helpers even though this would be something nice to have in general.
If the developer does not specify valid settings in app.yml (maybe via a typo), the developer does not get a fail message. In the case of helpers that are seldom used (e.g. error or exception handling), they may never find out about the problem until their app is out in the field and SURPRISE!

Example of how this might look:
(helper.yml file)

helper-options:
   output-target
      mandatory: false
      valid-values: msg,field,answer,,

The last set of double-commas means that the value can also be empty.

[trevordevore]

If this is something you want to take up, flesh out, and implement I can provide guidance.

[macMikey]

I figured this would be on me, so yes.