Add Passing Around And Using Modules as an elixir til

elixir/passing-around-and-using-modules.md

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+# Passing Around And Using Modules
+
+A module is a bag of functions. When we define a module, we are tying it to
+an atom. If we pass around the atom that references this module, then we can
+use it to call its functions.
+
+For example, consider two types of greetings:
+
+```elixir
+defmodule Hello do
+  def get_greeting do
+    "Hello, World!"
+  end
+end
+
+defmodule Hola do
+  def get_greeting do
+    "Hola, Mundo!"
+  end
+end
+```
+
+And a generic greeting module that accepts a language module:
+
+```elixir
+defmodule Greeting do
+  def say_hello(language_module) do
+    language_module.get_greeting
+    |> IO.puts
+  end
+end
+
+Greeting.say_hello(Hello) # => "Hello, World!"
+Greeting.say_hello(Hola) # => "Hola, Mundo!"
+```
+
+The module reference that we pass in to `Greeting.say_hello` can be used to
+invoke the `get_greeting` function.