Validation: adding multiple rule namespaces support

Allows an implementer to override definitions of rules, each added namespace takes priority over the default one

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)
Less client code, but also an optional setter is *also* optional. I'd rather reduce the testing surface of an exception vs. increase it in the caller