Add new koans for comprehensions

Added to partially complete the "Koans for special forms" issue.
https://github.com/elixirkoans/elixir-koans/issues/53

lib/koans/20_comprehensions.ex

@@ -0,0 +1,36 @@
+defmodule Comprehensions do
+  use Koans
+
+  @intro "Comprehensions"
+
+  # A comprehension is made of three parts: generators, filters and collectables.
+
+  koan "Generators provide the values to be used in a comprehension" do
+    # In the expression below, `n <- [1, 2, 3, 4]` is the generator.
+    assert (for n <- [1, 2, 3, 4], do: n * n) == ___
+  end
+
+  koan "Any enumerable can be a generator" do
+    assert (for n <- 1..4, do: n * n) == ___
+  end
+
+  koan "A generator specifies how to extract values from a collection" do
+    collection = [["Hello","World"], ["Apple", "Pie"]]
+    assert (for [a, b] <- collection, do: "#{a} #{b}") == ___
+  end
+
+  koan "You can use multiple generators at once" do
+    assert (for x <- [2, 4], y <- ["dogs", "cats"], do: "#{x} #{y}") == ___
+  end
+
+  koan "Use a filter to reduce your work" do
+    assert (for n <- [1, 2, 3, 4, 5, 6], n > 3, do: n) == ___
+  end
+
+  koan "Add the result of a comprehension to an existing collection" do
+    pies = ["Apple Pie"]
+    pies = for x <- ["Pecan", "Pumpkin"], into: pies, do: "#{x} Pie"
+    assert pies == ___
+  end
+
+end