Allow options to be passed to the regionType when building a custom region

When using a custom regionType using the
```javascript
{
  selector: "#custom",
  regionType: CustomRegion
}
```
syntax, it is not possible to pass extra configuration options to the custom region class.

This pull request allows you to pass extra options by simply including them into the region definition object. EG.
```javascript
{
  selector: "#custom",
  regionType: CustomRegion,
  doSomethingAwesome: true
}
```