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).

flag.go

@@ -14,13 +14,13 @@ import (
 const defaultPlaceholder = "value"
 
 // BashCompletionFlag enables bash-completion for all commands and subcommands
-var BashCompletionFlag = BoolFlag{
+var BashCompletionFlag Flag = BoolFlag{
 	Name:   "generate-bash-completion",
 	Hidden: true,
 }
 
 // VersionFlag prints the version for the application
-var VersionFlag = BoolFlag{
+var VersionFlag Flag = BoolFlag{
 	Name:  "version, v",
 	Usage: "print the version",
 }
@@ -28,7 +28,7 @@ var VersionFlag = BoolFlag{
 // HelpFlag prints the help for all commands and subcommands
 // Set to the zero value (BoolFlag{}) to disable flag -- keeps subcommand
 // unless HideHelp is set to true)
-var HelpFlag = BoolFlag{
+var HelpFlag Flag = BoolFlag{
 	Name:  "help, h",
 	Usage: "show help",
 }
@@ -630,7 +630,8 @@ func (f Float64Flag) ApplyWithError(set *flag.FlagSet) error {
 func visibleFlags(fl []Flag) []Flag {
 	visible := []Flag{}
 	for _, flag := range fl {
-		if !flagValue(flag).FieldByName("Hidden").Bool() {
+		field := flagValue(flag).FieldByName("Hidden")
+		if !field.IsValid() || !field.Bool() {
 			visible = append(visible, flag)
 		}
 	}
@@ -723,9 +724,8 @@ func stringifyFlag(f Flag) string {
 
 	needsPlaceholder := false
 	defaultValueString := ""
-	val := fv.FieldByName("Value")
 
-	if val.IsValid() {
+	if val := fv.FieldByName("Value"); val.IsValid() {
 		needsPlaceholder = true
 		defaultValueString = fmt.Sprintf(" (default: %v)", val.Interface())