Add bin/trigger, for testing event triggers

Summary:
Ref T6881. This makes it easier to fire a trigger and make sure it works properly. You can use the --now flag to travel through time, and test scheduling conditions with --last and --next. It will tell you when the trigger would reschedule.

Better than waiting 24 hours to see if things work.

Test Plan: Fired some backups, got useful output which made me think my code probably works correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11438