Use specs for autocomplete

Feature request. E.g. I have the following function:

``` erlang
-spec cmd(Command :: binary(), Params :: list()) -> {ok, Reply :: binary()} | error.
cmd(<<"help">>, []) ->
    % something goes here
cmd(<<"status">>, Message) ->
    % something goes here too
```

But it is autocompleted as `cmd(Param1, Message)`. Why don't use specs for readable parameter names if they're available?
