Introduce --koan=<name> to run a specific koan module.

test/options_test.exs

@@ -11,6 +11,11 @@ defmodule OptionTest do
     assert Options.clear_screen?
   end
 
+  test "can target specifc koans" do
+    Options.start(["--koan=Strings"])
+    assert Options.initial_koan() == Strings
+  end
+
   test "ignores unknown options" do
     Options.start(["--foo"])
     refute Options.clear_screen?