Add first arg `element` to `on`, `off`

[KODerFunk]

Hi @dgraham! I took into account your comment about the plans for typing (#22) and left only the possibility of specifying custom elements (not only document under hood) for listening.

I really need the ability to use different nodes for events delegation, as well as others (#15, #16, #20).

Do not rush to close, if something you do not like. Let's talk :)