If no action is specified on the command or app, print the help documentation

Rather than panic'ing or displaying an opaque error message about the
signature which is more confusing to the end user.

Fixes #562

app_test.go

@@ -178,6 +178,49 @@ func ExampleApp_Run_commandHelp() {
 	//    This is how we describe describeit the function
 }
 
+func ExampleApp_Run_noAction() {
+	app := App{}
+	app.Name = "greet"
+	app.Run([]string{"greet"})
+	// Output:
+	// NAME:
+	//    greet
+	//
+	// USAGE:
+	//     [global options] command [command options] [arguments...]
+	//
+	// COMMANDS:
+	//      help, h  Shows a list of commands or help for one command
+	//
+	// GLOBAL OPTIONS:
+	//    --help, -h     show help
+	//    --version, -v  print the version
+}
+
+func ExampleApp_Run_subcommandNoAction() {
+	app := App{}
+	app.Name = "greet"
+	app.Commands = []Command{
+		{
+			Name:        "describeit",
+			Aliases:     []string{"d"},
+			Usage:       "use it to see a description",
+			Description: "This is how we describe describeit the function",
+		},
+	}
+	app.Run([]string{"greet", "describeit"})
+	// Output:
+	// NAME:
+	//     describeit - use it to see a description
+	//
+	// USAGE:
+	//     describeit [arguments...]
+	//
+	// DESCRIPTION:
+	//    This is how we describe describeit the function
+
+}
+
 func ExampleApp_Run_bashComplete() {
 	// set args for examples sake
 	os.Args = []string{"greet", "--generate-bash-completion"}