Add WP CLI command to skip job backlog.

Adding a cron job with the code `wp_schedule_event( 0, 'daily', 'cavalcade_daily' );` will cause the job to run constantly until it's run once for each day since 1/1/1970. Once the job has _caught up_ it will run at midnight each day, as expected.

Adding a command such as `wp cavalcade job restart {id} --skip-backlog` will allow users to skip the backlog and ensure the job only runs once per day.

Follow up to humanmade/Cavalcade-Runner#53