implement custom proxy option per server #592

[oooo-ps]

Solution for #592

The logic is as follows:

1. If a server proxy is provided, it will be used.
2. If a server proxy is not provided, the global proxy will be used.
3. If the global proxy is not provided, a plain connection will be used.

The API is compatible with previous Halloy versions, proxy config for the specific server looks like that:

[servers.test]
nickname = "utest"
realname = "utest"
server = "irc.test.i2p"
port = 6667
use_tls = false
channels = []
[servers.test.proxy.http]
host = "xxx.xxx.xxx.xxx"
port = 4444

By the way, my rustfmt formatted main.rs. I hope that is okay with the modern Rust version.