Make tlsdate read proxy environment variables if present

[asbjorn]

Hi.

First of all - great tool!

I just used this tool on my laptop at work where we're sitting behind a corporate proxy. By default I've properly setup http_proxy, https_proxy, socks_proxy environment variables in my dotfiles. Since most linux tools read these variables I would expect tlsdate to do the same.

After reading the man pages I did notice the "-x <url>" option, but I still think that tlsdate should check the shell environment to try to auto-detect the proxy settings.