WIP Csrf plug

very rough
Tried doing:
```elixir
Mix.Config.persist(phoenix: [
  {Router, cookies: true, session_key: "_test_key", session_secret: "8Ja3xi2Tnr4=8Ja3xi2Tnr4=8Ja3xi2Tnr4=8Ja3xi2Tnr4=8Ja3xi2Tnr4=8Ja3xi2Tnr4="}
])
```

It doesn't trigger plugging of `Plug.Session` and `Plugs.SessionFetcher` so resorted to plugging them here, which is redundant since `use Phoenix.Router` should plug them, right?