Add support for mount point or express route config.

Currently, if someone chooses to use kraken with a particular root path, middleware will not be registered as expected.

This PR adds support for specifying a base route when using this module. Add middleware and routes will be mounted on the specified path.

This PR adds support for 2 ways of specifying mount point:
```javascript
app.use('/foo', kraken());
```
or
```json
{
    "express": {
        "route": "/foo"
    }
}
```