process specifiers (to add validations for required, etc.)(detect index modifications) decide what to do with multiple-column indices

add extensible specifiers:
  ModalFields.specify do
    required do |model, column|
       model.validates_presence_of column.name
    end
    unique do |model, column|
      model.validates_uniqueness_of name, :allow_nil => !column.specifiers.include?(:required)
    end
  end

rename hook to... filter? process? declared?  transformation?

complete field declaration validation

helper methods for field declaration: (can be used instead of the type and dispense with the need
of extra attributes)
  status enum_field(:draft, :approved, :published), :required
instead of
  status :enum_field, :required, :values=>[:draft, :approved, :published]