Add parameter validate setting

lib/tty/option/parameter.rb

@@ -88,6 +88,18 @@ def required?
         @settings.fetch(:required) { true }
       end
 
+      def validate(value = (not_set = true))
+        if not_set
+          @settings[:validate]
+        else
+          @settings[:validate] = value
+        end
+      end
+
+      def validate?
+        @settings.key?(:validate) && !@settings[:validate].nil?
+      end
+
       def to_sym
         self.class.name.to_s.split(/::/).last.downcase.to_sym
       end

spec/unit/parameter/argument_spec.rb

@@ -110,4 +110,21 @@
       expect(arg.optional?).to eq(true)
     end
   end
+
+  context "validate setting" do
+    it "returns nil" do
+      arg = described_class.new(:foo)
+
+      expect(arg.validate).to eq(nil)
+      expect(arg.validate?).to eq(false)
+    end
+
+    it "returns conversion value" do
+      validator = ->(val) { true }
+      arg = described_class.new(:foo, validate: validator)
+
+      expect(arg.validate).to eq(validator)
+      expect(arg.validate?).to eq(true)
+    end
+  end
 end