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 don't believe so. But why not use the app's logger? http://flask.pocoo.org/docs/0.10/quickstart/#logging