Refactors some aspects of `root_*` attributes
Refactors some aspects of `root_*` attributes

- makes the `allow_unknown` of the root-validator available
- set `root_*` attributes only for 1st generation
  - pass it to the following via `_config`
- don't validate `allow_unknown` in child validators

funkyfuture/cerberus,pyeve/cerberus,dkellner/cerberus,dkellner/cerberus,pyeve/cerberus,nicolaiarocci/cerberus,funkyfuture/cerberus,nicolaiarocci/cerberus