Add a config setting to log to a file

This logs all exception & stack traces by default.

If you want to log a warning, you can do something like:

```python
app.logger.warn("Something is bad!")
```

and it will show up in either the log file or the console depending on settings.
I'm not being clear; I am asking why we're treating logging in dev and in production differently. Is the only change here that in prod we don't emit INFO logs?