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.
Should do a similar check for the 'false' string as we have above for DEBUG, in case someone wants to set this to false in their config.