Idle interval between readings

This release adds a configurable idle interval between device readings to reduce file size.
Specify the amount of idle seconds in /etc/co2monitor/service.conf:

interval = 30

The service needs restarting for this to take effect. One of:

• sudo systemctl restart co2monitor
• qdbus --system de.nobodyinperson.co2monitor /de/nobodyinperson/co2monitor de.nobodyinperson.co2monitor.quit

and then re-plugging the device should do it.