List should have validation

Much like list_select.

    def list(&validation); LIST; end
    list_block = lambda do |input|
      args = validate(input, [Array]).collect do |arg|
        arg.kind_of?(String) ? YAML.load(arg) : arg
      end
      
      if validation
        args.collect! {|arg| validation.call(arg) }
      end
      
      args
    end