Allow setting notification timeout as CLI flag

I can't see any example in the readme of being able to do this. I would like to simply be able to do this: `ntfy --timeout=0 done sleep 3` and have the timeout be disabled for desktop notifications.

It's quite inflexible if this can only be done via the config file as this flag would be very useful to include for instance when you know that you need to leave the computer for a while.