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.
Seems like we'd want this to replace the default logging in dev, as we'll want to see output from `logger.X` while developing as well.