CLI: Support nested subcommand

[JunichiSugiura]

#112

• Subcommand -> SubcommandPlugin
• generated subcommand plugin needs to be manually added
• add subcommand name suffix to each event name

#[derive(SubcommandPlugin, clap::Subcommand, Clone)]
pub enum Action {
    // Named variant
    Hello { name: Option<String> },
    // Unnamed
    Hello2(Hello2Args),
    // Unit
    Ping,
    
    #[clap(subcommand)]
    Task(TaskAction)
}

#[derive(SubcommandPlugin, clap::Subcommand, Clone)]
pub enum TaskAction {
    Install,
}

// Generated Events

// HelloAction
// Hello2Action
// PingAction
// TaskAction
// InstallTaskAction