Add restart subcommand

[yottahmd]

The user should be able to restart a DAG execution. The restart command stops the current execution and starts the DAG after a certain pause time for cleanup. If the DAG is not running, it just starts the DAG.

dagu restart ./dags/task.yaml

The cleanup time for restart should be configured in the DAG definition. The default value is 0.

RestartWaitSec: 30

[yottahmd]

Need to add restart key to schedule field also:

schedule:
  start: "0 8 * * *"
  restart: "0 12 * * *"
  stop: "0 23 * * *"