Support @reboot command

Hi there,

I'm currently running supercronic in a docker and it's working great.

I would like to run my script on "boot" e.g. when supercronic is started, as well as at a set interval. Normally in many cron versions we can use the standard @reboot to run a script on reboot, but in my testing it looks like supercronic doesn't currently support this.

Would it be possible to add this functionality? I notice that other shortcuts such as @hourly are supported.

I do have a work-around which is to run a shell script /start-container which simply runs my script and then runs supercronic, but it seems a bit awkward!

Start by tracing where the existing @hourly shortcut is parsed and where scheduling begins when supercronic starts. Confirm the expected @reboot behavior in the Docker use case, then add coverage alongside the existing shortcut tests so the startup trigger and its completion behavior are verifiable.

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