Allow configuration of MIX_ENV

SuperElixir version 0.2.4

What's new?

Now you can configure the MIX_ENV, by using the mix_env entry in your project setting:

# <project name>.sublime-project
{
    // ...

    "settings": {
        // ...
        "elixir_interpreter": "~/elixir-2.4.4/bin/elixir",
        "mix_env": "test",
    }
}