Allow `iex -S mix` for `elixir-iex-command`

[artisonian]

When following examples from Elixir's getting started guide, it's useful to start IEx using iex -S mix.

[mattdeboard]

@artisonian It looks like elixir-mix provides this functionality in a discrete package (vice being glommed into elixir-mode). I'm going to close this out as it looks as though this feature request might be better suited there.

If you think this isn't the case please go ahead and re-open the issue. Thanks!