Add before function to app

app.go

@@ -20,6 +20,9 @@ type App struct {
 	Commands []Command
 	// List of flags to parse
 	Flags []Flag
+	// An action to execute before any subcommands are run, but after the context is ready
+	// If a non-nil error is returned, no subcommands are run
+	Before func(context *Context) error
 	// The action to execute when no subcommands are specified
 	Action func(context *Context)
 	// Compilation date
@@ -93,6 +96,13 @@ func (a *App) Run(arguments []string) error {
 		return nil
 	}
 
+	if a.Before != nil {
+		err := a.Before(context)
+		if err != nil {
+			return err
+		}
+	}
+
 	args := context.Args()
 	if args.Present() {
 		name := args.First()