Add new item to control contacts (ContactFilter)

One more item to control over contacts.
It's very simple one< like that:

```javascript
ContactFilter {
     world: world
     onContacted: {
         if( (fixtureA. == fixtureB)
             contactFilter.result = false;
        }
}
```
so id contactFilter.result == false all contacts with these two fixturess are ignored.

I will add test code to contacts example later if this path will be approved.