Add daemon mode #10
Description
Add a daemon mode that keeps the program running and periodically updating DNS. A new flag, --daemon, will specify how often DNS should refresh. It will take a required argument of a Go-style duration, e.g. 10m, 1hr, etc, as defined here. The env var DAEMON can be set to achieve the same effect, with its value set to the duration, e.g. DAEMON=15m.