Move sigils to later in the lessons

While doing this, I also discovered that
there is also a reference to lists in numbers, but
that might a bit easier for someone to grasp, given
the hint that the koan gives, and the output they
see when they run it.

lib/koans/07_keyword_lists.ex

@@ -0,0 +1,47 @@
+defmodule KeywordLists do
+  use Koans
+
+  @intro "KeywordLists"
+
+  koan "Like maps, keyword lists are key-value pairs" do
+    kw_list = [foo: "bar"]
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "Keys may be repeated, but only the first is accessed" do
+    kw_list = [foo: "bar", foo: "baz"]
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "You could access a second key by removing the first" do
+    kw_list = [foo: "bar", foo: "baz"]
+    [_|kw_list] = kw_list
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "Keyword lists are just special syntax for lists of two-element tuples" do
+    assert [foo: "bar"] == [{___, ___}]
+  end
+
+  koan "But unlike maps, the keys in keyword lists must be atoms" do
+    not_kw_list = [{"foo", "bar"}]
+
+    assert_raise ArgumentError, fn -> not_kw_list[___] end
+  end
+
+  koan "Conveniently keyword lists can be used for function options" do
+    transform = fn str, opts ->
+      if opts[:upcase] do
+        String.upcase(str)
+      else
+        str
+      end
+    end
+
+    assert transform.("good", upcase: true) == ___
+    assert transform.("good", upcase: false) == ___
+  end
+end