Append or insert settings.

Sometimes I need to add a middleware on the top or near the top.
An example is the Debug Toolbar.

```
The order of MIDDLEWARE_CLASSES is important. You should include the Debug Toolbar 
middleware as early as possible in the list. However, it must come after any other middleware that 
encodes the response’s content, such as GZipMiddleware.
```

It's possible with logan? 
