Simpler progressbar and handle --koan=<KOAN> better

lib/runner.ex

@@ -17,19 +17,13 @@ defmodule Runner do
   def koan?(koan), do: Enum.member?(@modules, koan)
   def modules, do: @modules
 
-  def run do
-    Options.initial_koan
-    |>run
-  end
+  def modules_to_run, do: Options.initial_koan |> modules_to_run
+  def modules_to_run(start_module), do: Enum.drop_while(@modules, &(&1 != start_module))
 
-  def run(start_module) when start_module in @modules, do: run(start_module, @modules)
-  def run(start_module), do: Display.invalid_koan(start_module, @modules)
-
-  def run(start_module, modules) do
+  def run(modules) do
     Display.clear_screen()
 
     modules
-    |> Enum.drop_while( &(&1 != start_module))
     |> Enum.take_while( &(run_module(&1) == :passed))
   end
 
@@ -38,7 +32,6 @@ defmodule Runner do
     |> Display.considering
     |> Execute.run_module(&track/2)
     |> display
-
   end
 
   defp track(:passed, koan), do: Tracker.completed(koan)