Improve and enforce naming of koans at compile time

test/support/passing_koan.ex

@@ -1,7 +1,7 @@
 defmodule PassingKoan do
   use Koans
 
-  koan "hi there" do
+  koan "Hi there" do
     assert 1 == 1
   end
 end

test/support/sample_koan.ex

@@ -1,7 +1,7 @@
 defmodule SampleKoan do
   use Koans
 
-  koan "thinking more than once" do
+  koan "Thinking more than once" do
     assert 3 == :__
     assert 4 == :__
   end