manual scroll will cancel jump

[artemdemo]

This is solution for issue #7

I added new option:

Jump.jump('.selector', {
  scrollCancelJump: true
})

If true jump will be cancelled and user will be able to continue scrolling with the mouse or touchpad or whatever.

[callmecavs]

going to take a pass on this - I think there may be an easier way to implement it, and I'm not sure I want to do so yet. thanks for the contribution though