Add ability to use custom Flag types

Users can now use custom flags types (conforming to the Flag interface)
in their applications. They can also use custom flags for the three
global flags (Help, Version, bash completion).

app_test.go

@@ -1520,6 +1520,63 @@ func TestApp_OnUsageError_WithWrongFlagValue_ForSubcommand(t *testing.T) {
 	}
 }
 
+// A custom flag that conforms to the relevant interfaces, but has none of the
+// fields that the other flag types do.
+type customBoolFlag struct {
+	Nombre string
+}
+
+// Don't use the normal FlagStringer
+func (c *customBoolFlag) String() string {
+	return "***" + c.Nombre + "***"
+}
+
+func (c *customBoolFlag) GetName() string {
+	return c.Nombre
+}
+
+func (c *customBoolFlag) Apply(set *flag.FlagSet) {
+	set.String(c.Nombre, c.Nombre, "")
+}
+
+func TestCustomFlagsUnused(t *testing.T) {
+	app := NewApp()
+	app.Flags = []Flag{&customBoolFlag{"custom"}}
+
+	err := app.Run([]string{"foo"})
+	if err != nil {
+		t.Errorf("Run returned unexpected error: %v", err)
+	}
+}
+
+func TestCustomFlagsUsed(t *testing.T) {
+	app := NewApp()
+	app.Flags = []Flag{&customBoolFlag{"custom"}}
+
+	err := app.Run([]string{"foo", "--custom=bar"})
+	if err != nil {
+		t.Errorf("Run returned unexpected error: %v", err)
+	}
+}
+
+func TestCustomHelpVersionFlags(t *testing.T) {
+	app := NewApp()
+
+	// Be sure to reset the global flags
+	defer func(helpFlag Flag, versionFlag Flag) {
+		HelpFlag = helpFlag
+		VersionFlag = versionFlag
+	}(HelpFlag, VersionFlag)
+
+	HelpFlag = &customBoolFlag{"help-custom"}
+	VersionFlag = &customBoolFlag{"version-custom"}
+
+	err := app.Run([]string{"foo", "--help-custom=bar"})
+	if err != nil {
+		t.Errorf("Run returned unexpected error: %v", err)
+	}
+}
+
 func TestHandleAction_WithNonFuncAction(t *testing.T) {
 	app := NewApp()
 	app.Action = 42
@@ -1642,7 +1699,7 @@ func TestShellCompletionForIncompleteFlags(t *testing.T) {
 
 		for _, flag := range ctx.App.Flags {
 			for _, name := range strings.Split(flag.GetName(), ",") {
-				if name == BashCompletionFlag.Name {
+				if name == BashCompletionFlag.GetName() {
 					continue
 				}
 
@@ -1659,7 +1716,7 @@ func TestShellCompletionForIncompleteFlags(t *testing.T) {
 	app.Action = func(ctx *Context) error {
 		return fmt.Errorf("should not get here")
 	}
-	err := app.Run([]string{"", "--test-completion", "--" + BashCompletionFlag.Name})
+	err := app.Run([]string{"", "--test-completion", "--" + BashCompletionFlag.GetName()})
 	if err != nil {
 		t.Errorf("app should not return an error: %s", err)
 	}