Ziggy should support automation/notification

[vanhauser-thc]

automation:

• If cargo ziggy build fails then send a notification
• If cargo ziggy run fails then send a notification
• If a crash is found, send a notification (once per day only, collect them)
• If no coverage is found for X minutes, send a notification (once per day)

Notification: execute a specified OS command.

Maybe something like:

-B command
-R command
-C command
-S 30:command  (stalled coverage for 30 minutes: execute command once per day)