Validation: adding multiple rule namespaces support

Also: 
- reduced complexity/customization of "matching" rule
- added extension points for downstream modification
- allow `->setRule()` to accept rules in an array vs. just piped-delimited
- added run-completion logic to prevent retrieving pre-validated data
- Swapped to proper PSR-4 namespace (added vendor)
but still, do we need the setter? I guess the only justification would be to assert the case on the constructor, right? Otherwise, I see these private setters as unnecessary.