title: Issue with simple elixir/mix project
body: <p>Im learning Elixir and trying to compose a SIMPLE elixir hello world example but I keep running into this error when I do "mix run"</p>
  
  <pre><code> (Mix) Could not start application collector:
         Collector.start(:normal, []) returned an error:
         shutdown: failed to start child: Collector.Sample
  (EXIT) nil
  </code></pre>
  
  <p>What am I doing wrong? Here is my code minus the mix.exs and other non-critical files</p>
  
  <pre><code>defmodule Collector do
    use Application
  
    def start(_type, _args) do
      Collector.Supervisor.start_link
    end
  end
  </code></pre>
  
  <hr>
  
  <pre><code>defmodule Collector.Supervisor do
    use Supervisor
  
    def start_link do
      Supervisor.start_link(__MODULE__, :ok)
    end
  
    def init(:ok) do
      children = [
        supervisor(Collector.Sample, [])
      ]
  
      supervise(children, strategy: :one_for_one)
    end
  end
  </code></pre>
  
  <hr>
  
  <pre><code>defmodule Collector.Sample do
    use Application
  
    def start_link do
  
    end
  
    def run do
      IO.puts "Hello World"
    end
  
  end
  </code></pre>
  
  <p>I just wanna dump "Hello World" when I run "mix run" in the console and I dont know how or where to call the <code>run()</code> method. Thank you</p>