Some Features For Your Consideration

This pull request has a couple things in it.  You may only want one of them.  Or neither of them.  :-)

The first allows storing validation errors in the flash.  Useful if you use the Post-Redirect-Get pattern.  But since all the other plugins (like tag helpers) also use the stash, I don't actually use that pattern myself anymore.

The second is a callback to dynamically name the validation class.  This lets you create your own rules for naming controllers & actions & validators.  I think it is pretty clever, but may not be a good idea.

I also added tests that should cover all the changes I made.