Pass request context to handler

[sternenseemann]

Would break library interface unfortunately, but having something like

data GopherRequest
  = GopherRequest
  { grSelector :: ByteString
  , grAddress :: (Word16, Word16, Word16, Word16, Word16, Word16, Word16, Word16)  -- ipv6 address
  }

Could be very useful for applications which want to kind of differentiate different users.