Make messages configurable

[htuscher]

I think this was the intention but as you're not passing an atom to Keyword.get, it makes it impossible to override the message.

A gettext integration would be highly appreciated, but for now, this should be sufficient.

[dennym]

You can pass in the option :message into the validate_date/3 and it will be used since the :message atom gets defaulted by message/3 at lib/validators/date.ex#L147. Its a bit unusual and confusing but it works.