Add `validate_uniqueness_of_(:foo).allow_blank`

Complements [`validates_uniqueness_of(:foo, allow_blank: 
true)`](http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of).
Would it be good to merge these two booleans together?

`if (@options[:allow_nil] && existing_value.nil?) || (@options[:allow_blank] && existing_value.blank?)`