Ensure that watcher does not try to run a module that is not a koan

lib/runner.ex

@@ -14,6 +14,10 @@ defmodule Runner do
     Tasks,
   ]
 
+  def koan?(module) do
+    Enum.member?(@modules, module)
+  end
+
   def run do
     Options.initial_koan
     |>run

lib/watcher.ex

@@ -4,8 +4,10 @@ defmodule Watcher do
   def callback(file, events)  do
     if Enum.member?(events, :modified) do
       try do
-        [{mod, _} | _] = Code.load_file(file)
-        Runner.run(mod)
+        Code.load_file(file)
+          |> Enum.map(&(elem(&1, 0)))
+          |> Enum.find(&Runner.koan?/1)
+          |> Runner.run
       rescue
         e -> Display.show_compile_error(e)
       end