add --sleep argument

[pietdevaere]

Add a flag --sleep that

• is ignored if quota is available on one of the accounts, but
• otherwise makes the application sleep until quota is available again.

In combination with the postcards-folder plugin his would make it very easy to run the program as a systemd service using Restart: always. This is superior over using cron as it would eliminate the time between the quota becoming available and the next execution of the cron job.

[pietdevaere]

Might implement this myself, I assume this should be easy to implement.