[Feature Request] Set different upstream servers for every listening address

[baby9]

Usually, I set 8.8.8.8 and 8.8.4.4 as main DNS servers. When connections are unstable, it will switch to backup DNS servers.

./dnsproxy -l 127.0.0.1 -p 53 -u 8.8.8.8:53 -u 8.8.4.4:53
./dnsproxy -l 127.0.0.2 -p 53 -u 1.1.1.1:53 -u 1.0.0.1:53

Like the above, I have to start dnsproxy twice. I hope in the future we can specify upstream servers for each listening address in one command line.