Add support for route argument when mounting meddleware.

Currently, if someone chooses to use meddleware with a particular root path, routes will not resolve correctly.

This PR adds support for specifying a base route when using this module.
```javascript
app.use('/foo', meddleware(config));
```
and effectively is the same to doing
```javascript
app.use('/foo', express.cookieParser());
// etc...
```