working ipv6 : allow single atom opt and inet6 in particular

Hi Benoit :)
When I try to connect to an ipv6 host with hackney, I currently get the exception :

    {function_clause,
                         [{hackney_util,filter_options,
                              [[inet6],
                               [linger,nodelay,keepalive,send_timeout,
                                send_timeout_close,raw],
                               [binary,
                                {active,false},
                                {packet,raw},
                                {nodelay,true}]],
                              [{file,"src/hackney_util.erl"},{line,21}]},
                          {hackney_tcp_transport,connect,4,
                              [{file,"src/hackney_tcp_transport.erl"},
                               {line,32}]},
                          {hackney_connect,do_connect,4,
                              [{file,"src/hackney_connect.erl"},{line,214}]},
                          {hackney_connect,connect,5,
                              [{file,"src/hackney_connect.erl"},{line,30}]}

So I made two little changes in order to make hackney working with ipv6 :

- add single atom option management in `hackney_util:filter_options`
- add inet6 in accepted options of ssl and tcp sockets.