Group the updated domain into one single notification

[jemyzhang]

• group the subdomains into one single notification to reduce the duplicated ip updating message

[jemyzhang]

internal/provider/dreamhost/dreamhost_handler.go:50:2: if-return: redundant if ...; err != nil check, just return error instead. (revive)
    if err := provider.updateDNS(lastIP, currentIP, hostname, "add"); err != nil {
        return err
    }
internal/utils/ip.go:146:10: superfluous-else: if block ends with a continue statement, so drop this else and outdent its block (revive)
        } else {
            log.Debugf("get ip success by: %s, online IP: %s", reqURL, onlineIP)
            break
        }
pkg/notification/email.go:41:2: if-return: redundant if ...; err != nil check, just return error instead. (revive)
    if err := d.DialAndSend(m); err != nil {
        return err
    }