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.someflag == fixtureB.someflag)
             contactFilter.result = false;
        }
}
```
so id `contactFilter.result == false` all contacts with these two fixtures are ignored.

I will add test code to `contacts` example later if this path will be approved.