feat(body): add isTrigger option

[age2pierre]

Hello,
I did this small fork to add the "sensor" option to bodies, the original feature-request is referenced here => schteppe#146 and all the credit for the actual code goes to => schteppe#424 ;)
I've committed the dist/ folder to test without publishing the package, but I can drop it from the PR if needed.

[marcofugaro]

Thanks for the PR @age2pierre, I think this feature could be useful for many users.

I've committed the dist/ folder to test without publishing the package, but I can drop it from the PR if needed.

Please exclude it from the PR, otherwise, there will be a lot of conflicts if we build on master 😬

Some considerations:

1. I think isTrigger would be a better name for it. sensor is used in a 2d game engine environment, while 3d game engines use trigger: triggers in Unity, triggers in Unreal. Bullet calls them GhostObjects 🤷‍♂️.
2. It would be better to have a trigger.html example instead of adding the code in the collisions.html. The trigger would need to be rendered with a wireframe style, just like wireframeMaterial but green or yellow:

this.triggerMaterial = new THREE.MeshBasicMaterial({ color: 0x00ff00, wireframe: true })

3. The example should show the usage of at least the beginContact and endContact events. Unity has also the OnTriggerStay event, but maybe it's better to do that in a future PR since it's a new event for cannon.

[age2pierre]

@marcofugaro I've updated based upon your review and rebased upon master.
I've kept the demo still very minimal, tell me if you fancy something fancier :)

[marcofugaro]

Thanks! I adjusted the demo a little bit, opened a new PR with also your commits since I wasn't allowed to edit this one: #83