Support proxies

[soywod]

Here how I see the thing: every time we build a TPC connection (IMAP and SMTP), we need to apply the right proxy according to given proxy URL. It can be HTTP(S), SOCKS4 or SOCKS5. The proxy URL is taken from http_proxy and https_proxy env vars by default, and we can add a global config entry proxy-url or proxy that overrides this behaviour. Here an example on how to set up the socks proxies, and here an example on how to use the HTTP ones.