Add configuration settings for whether to use IPv4 and/or IPv6#169

- Possible setting names: `inet.ipv4` and `inet.ipv6`. - The value is a boolean. - Alternatively, add one setting, `inet.ip-version`, which can be `4`, `6`, or `0` (for any). - This can be implemented using the following methods: - HTTP tracker connections: Call `reqwest::ClientBuilder::local_address()` with an IP address of "0.0.0.0" or "::"? - UDP tracker connections: Call `tokio::net::UdpSocket::bind()` with an IP address of "0.0.0.0" or "::"? - How would this work when then attempting to connect to a domain name with only incompatible IPs? - Peer connections: `tokio::net::TcpSocket::new_v4()` vs. `tokio::net::TcpSocket::new_v6()` - Try to filter out incompatible IP addresses in advance instead of letting the connection attempts fail?