Include extension migration generation mix task

This module name contains `Isn` (not `ISN`), because otherwise mix will
not find it when running `mix isn.gen.migration`.

lib/tasks/isn.gen.migration.ex

@@ -0,0 +1,51 @@
+defmodule Mix.Tasks.Isn.Gen.Migration do
+  use Mix.Task
+  import Mix.Ecto
+  import Mix.Generator
+
+  @migration_name "CreateISNExtension"
+
+  @shortdoc "Generate an Ecto migration to add the `isn` extension"
+
+  @moduledoc """
+  Generate an Ecto migration to add the `isn` extension to your database.
+
+  ## Example
+
+      mix isn.gen.migration
+  """
+
+  @doc false
+  def run(args) do
+    Mix.Task.run "app.start", args
+    [repo] = parse_repo(args)
+    filename = "#{timestamp}_create_isn_extension.exs"
+    IO.inspect([repo, Mix.Project.app_path, filename])
+    path = Path.relative_to(migrations_path(repo), Mix.Project.app_path)
+    file = Path.join(path, filename)
+    create_directory(path)
+    mod = Module.concat([repo, Migrations, @migration_name])
+    create_file(file, migration_template(mod: mod))
+  end
+
+  defp timestamp do
+    {{y, m, d}, {hh, mm, ss}} = :calendar.universal_time()
+    "#{y}#{pad(m)}#{pad(d)}#{pad(hh)}#{pad(mm)}#{pad(ss)}"
+  end
+
+  defp pad(s), do: to_string(s) |> String.rjust(2, ?0)
+
+  embed_template :migration, """
+  defmodule <%= inspect @mod %> do
+    use Ecto.Migration
+
+    def up do
+      execute "CREATE EXTENSION IF NOT EXISTS isn;"
+    end
+
+    def down do
+      execute "DROP EXTENSION IF EXISTS isn;"
+    end
+  end
+  """
+end