Add a `Validator` base class and convert all built-in validators to classes. Also move __all__ to the top. This still needs more work, particularly the documentation.
Add a `Validator` base class and convert all built-in validators to classes. Also move __all__ to the top. This still needs more work, particularly the documentation.

maxcountryman/wtforms,mfa/wtforms-clone,mfa/wtforms-clone