Add very basic database migration helpers.

For example, you can do Ecto migrations by running
$ _build/prod/rel/runosaari/bin/runosaari eval "Runosaari.Release.migrate"

lib/runosaari/release.ex

@@ -0,0 +1,24 @@
+defmodule Runosaari.Release
+  @app :runosaari
+
+  def migrate do
+    load_app()
+
+    for repo <- repos() do
+      {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :up, all: true))
+    end
+  end
+
+  def rollback(repo, version) do
+    load_app()
+    {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :down, to: version))
+  end
+
+  defp repos do
+    Application.fetch_env!(@app, :ecto_repos)
+  end
+
+  defp load_app do
+    Application.load(@app)
+  end
+end