added BodyComp#multiJump() to do multiple jumps easy

[lajbel]

I think it's great that you can do multiple jumps in an easy way, but also if someone needs to do it manually, they can always use jump.

Also added Body#onMultiJump() and on("multiJump") events.

[slmjkdbtl]

I'm actually also thinking about separating doubleJump (and possibly multijump) to its own component like before, to teach more about components, wdyt?

[lajbel]

would serve as an example, it would be great

[lajbel]

closed in favor of doubleJump comp