[day-02] add context to create/get a user

Create context module to create and fetch a user.

lib/wabanex/users/create.ex

@@ -0,0 +1,9 @@
+defmodule Wabanex.Users.Create do
+  alias Wabanex.{Repo, User}
+
+  def call(params) do
+    params
+    |> User.changeset()
+    |> Repo.insert()
+  end
+end