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)
I know this was merged in a previous PR, but what about renaming `NoSuchRuleException`  to `UnknownRuleException`, sounds a bit more standard?