Automatic DNS record removal option

When using the single liner like seen here https://developers.cloudflare.com/cloudflare-one/tutorials/single-command
it would be really nice if there was an extra flag that could be passed to delete the DNS record on exit. Maybe something like `--clean` so that when the `cloudflared` execution ends the DNS record is deleted.

This is useful for when you have relatively or very ephemeral tunnels that have unique subdomains.