velocity keeps high if move fast, then not end

Nobody has claimed this yet.

I just came across this package. Really useful!

But there is one behavior that's not expected in my opinion:

I call `spring.start` in `mousedown`, `spring.move` in `mousemove`, `spring.end` in `mouseup` and `spring.update` in `requestAnimationFrame`.

Now if I press the mouse button, then move the mouse really fast, then stop the mouse movement abruptly but hold the button pressed the `spring.value` stops changing also, but if I now release the mouse button the `spring.value` starts changing again because it has still a height `velocity` value that is only being used while `spring.interacting` is `false`.

The reason is obviously because there is no `mousestop` event in javascript that could be called if the mouse is _not_ moving anymore.

My current solution is the following:

``` ts
let framesSinceMove = 0;
function onDragMove(evt) {
framesSinceMove = 0;
spring.move(evt.clientX)
}

function onDragEnd(evt) {
spring.end(evt.clientX)
}

function tick() {
if(spring.interacting && ++framesSinceMove > 3) {
spring.velocity = 0
}
spring.update()
}
```

each time `spring.update` is called I increment a counter. Each time the mouse is moved I reset the counter. Then if `spring.interacting` is `true` and `update` has been called more than 3 times (ie the mouse has not been moved for 4 frames) I reset the velocity.

I would suggest to build this into the library itself. Would you accept a pull request?

Start at the spring.start, spring.move, spring.end, and spring.update entry points described in the issue, focusing on how velocity is retained while spring.interacting is true. Reproduce the fast-move-then-hold sequence and verify that releasing the mouse does not restart motion caused by stale velocity.

Written by the indexing model from the issue text.