add support for socks5 user and pass parameters.

[skie]

Hackney supports the connection via a socks5 proxy. To set a socks5 proxy, it uses the following settings:

• {proxy, {socks5, ProxyHost, ProxyPort}}: to set the host and port of the proxy to connect.
• {socks5_user, Username}: to set the user used to connect to the proxy
• {socks5_pass, Password}: to set the password used to connect to the proxy

This info from hackney documentation.

This PR adding support of socks5_user and socks5_pass.

[edgurgel]

Hey thanks for the PR. Could you please add a test similar to this one to check for this option?

https://github.com/edgurgel/httpoison/blob/master/test/httpoison_base_test.exs#L121-L133

[skie]

Testcase added. Additionally i added example how propertly define socks5 proxy using tuple. It was not clear for me that I must use single quotes for hostname.

[edgurgel]

Thanks!