Save history of jobs in persistent storage

[frndmg]

It could be useful to store the history of executed (or failed tasks) into persistent storage (SQLite, etc). For example, to restore previously broken tasks. Or just to automatically keep an eye on previously executed tasks.

[sm4ll-3gg]

This pull request is preparation for storing schedule inside external storage: #251

[robfig]

I think this is possible today by creating a custom JobWrapper that records this information for each execution. You can include it using cron.WithChain when creating your cron instance