Add dialyzer and type specs to `imap` modules

[emilianobovetti]

Hello! I'd like to contribute to the imap client (looking forward to implement the append command), but I'd love to document some of the internals first.

This pr introduces dialyzer/dialyxir and some typespecs to imap** modules. At this point mix dialyzer points out some issues:

• lib/mailroom/imap.ex:124:pattern_match
  The pattern can never match the type.
  
  Pattern:
  {:error, _reason}
  
  Type:
  {:ok, binary()}
  

  it complains about this pattern, because of the login spec, I'm not sure if currently GenServer.call(pid, {:login, username, password}) can actually reply with an {:error, binary}, but I'm not super sure about that.

• lib/mailroom/imap.ex:309:guard_fail
  The guard clause:
  
  when _ :: :ok === nil
  
  can never succeed.
  

  that's because the GenServer.cast at line 309 should always return :ok, so I think it complains the && isn't helpful

• lib/mailroom/imap/utils.ex:102:pattern_match
  The pattern can never match the type.
  
  Pattern:
  {nil, _rest}, false, _acc
  
  Type:
  nil | {binary(), binary()}, boolean(), [binary()]
  

  it looks like do_parse_string({nil, rest}, false, acc) can never match because String.next_grapheme("") == nil, I believe this one could be fixed by just matching nil

I'd like to address these issues in other pull requests, in this way we keep only typespecs and non-functional code in this one.

Let me know if looks good, cheers!