#9 Override default ValidationPerformers

[globalbus]

That's a part of my fork of this project.
https://bitbucket.org/globalbus/html5-validator-dialect-fork
I added a possibility to override any of previously added ValidationPerformers

Everything is on branch "performersOverride". It's a ValidationPerformerFactory rewrite to use Map instead of List, to keep performers. Also unit tests was patched for this solution (ValidationPerformers mocks must return valid constraintClass).