Enforce strict config unmarshalling about go-flow-levee HOT 2 CLOSED

@mlevesquedion Does this basically boil down to these rules?
If it's a funcMatcher, then only allow package, packageRE, receiver, receiverRE, method, methodRE.
If it's a sourceMatcher, then only allow package, packageRE, type, typeRE, field, fieldRE.

from go-flow-levee.

Those would be the main rules to enforce, yes! We would also like the matching for FieldTags to be strict:

FieldTags:
  - Key: levee
    Val: source
    SomeOtherKey: oops, this shouldn't be allowed

More generally, anywhere that the config yaml contains a field that we can't represent in our internal config, that's an error.

from go-flow-levee.