Add support for ExtraInfo.

app.go

@@ -85,6 +85,8 @@ type App struct {
 	ErrWriter io.Writer
 	// Other custom info
 	Metadata map[string]interface{}
+	// Carries a function which returns app specific info.
+	ExtraInfo func() map[string]string
 	// CustomAppHelpTemplate the text template for app help topic.
 	// cli.go uses text/template to render templates. You can
 	// render custom help text by setting this variable.