Get lessons order from filenames. Fix #196

test/runner_test.exs

@@ -0,0 +1,9 @@
+defmodule RunnerTest do
+  use ExUnit.Case, async: true
+
+  test "path to number" do
+    path = "lib/koans/01_just_an_example.ex"
+    assert Runner.path_to_number(path) == 1
+  end
+
+end