Cannot use this with ports

Hello, this is an awesome package, unfortunately I can't use this because I have outgoing ports, and ports can only return `Cmd a` or `Sub a`, using `Testable.Cmd.Cmd a` throws an error:

``` elm
port requestDownloadHead : HeadHash -> Testable.Cmd.Cmd a
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You are saying it should be:

Account.Model.HeadHash -> Testable.Cmd.Cmd a

But you need to use the particular format described here:

```

And although I don't need elm-testable to test outgoing ports (because I can do [like this](https://github.com/rogeriochaves/elm-peer-tweet/blob/90d6f5cdcdfdedb1a1decb3ee30d6d4d673dac32/test/Download/CmdSpec.elm#L23)), I want to mix Cmds for ports and tasks and still be able to test them.

My suggestion would be to add an extra type for Testable.Cmd, like `PortCmd`

Start by reading the Elm effect managers guide and inspecting Testable.Cmd.Cmd, then compare the linked Download/CmdSpec.elm example. Done means outgoing port commands can be represented alongside task commands without the type error, while remaining testable.

Written by the indexing model from the issue text.