Add basic file watcher to run koans

lib/watcher.ex

@@ -0,0 +1,10 @@
+defmodule Watcher do
+  use ExFSWatch, dirs: ["lib/koans"]
+
+  def callback(file, events)  do
+    if Enum.member?(events, :modified) do
+      [{mod, _}] = Code.load_file(file)
+      Runner.run(mod)
+    end
+  end
+end