Remove deprecated word_limit option
Fix custom validation conditional

Custom validations should only execute when there are no existing errors on the
associated field. Checking that the field has a value is not sufficient because
that only covers one kind of error (required fields not having a value).
Eliminate Formeze::Label class

No need to use a class when just a method will do. This also makes it a
bit easier to monkey-patch the algorithm for generating default labels.
Tweak README explanation of error messages
Bump version to v2.1.0