Make it easier to add env specific dependencies/applications

I just integrated `dotenv_elixir` into my phoenix project. This made me realise that environment specific dependencies are pretty much standard. It should be even easier to specify them. This is also suggested here: http://elixir-lang.org/getting_started/mix_otp/1.html#1.4-environments

The suggested change allows me to simply add this:

```elixir
defp deps(:dev), do: [{:dotenv, "~> 0.0.4"} | deps(:prod) ]
```

Similarly:

```elixir
defp app_list(:dev), do: [:dotenv | app_list(:prod)]
```

I know this is a super minor change and I'm not even sure this warrants a PR but I was desperate to contribute in some way. :smile: