add cron-schedule support to alerts by changing IsEnabled result

[umputun]

The change is back-compatible. It suppresses alerts outside the user-defined crontab format schedule by setting enabled to false.

The goal is to suppress alerts with user-provided crontab formatted settings. It is somewhat similar to the current maintenance but allows more flexibility in setting the schedule. For example, if cron-schedule is set to * 9-18 * * the alert will be disabled before 9:00 and after 18:59. I think, adding it on the Alert level is the simplest solution here with the minimal code change.

See discussion #250.

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Added the documentation in README.md, if applicable.

Note: I'm not sure what is the proper section in the readme to add this info