Add socks5 proxy support for TOR usage

[mdosch]

Thank you for this useful program. As it is probably used by a lot of privacy aware people I think it would be useful to provide socks5 proxy support to use it over an proxy, e.g. TOR.

[mdosch]

Some more thoughts: Maybe privatebin could honor the env variable HTTP-PROXY but also a proxy flag to explicitely set a proxie. The flag should override the env variable.