Add an agent to keep track of completed koans

lib/tracker.ex

@@ -0,0 +1,24 @@
+defmodule Tracker do
+  def start(modules) do
+    total = modules
+            |> Enum.flat_map(&(&1.all_koans))
+            |> Enum.count
+
+    Agent.start_link(fn -> {total, MapSet.new()} end, name: __MODULE__)
+  end
+
+  def get do
+    Agent.get(__MODULE__, &(&1))
+    |> summirize
+  end
+
+  def completed(koan) do
+    Agent.update(__MODULE__, fn({total, completed}) ->
+      {total, MapSet.put(completed, koan)}
+    end)
+  end
+
+  def summirize({total, completed}) do
+    %{total: total, current: MapSet.size(completed)}
+  end
+end