A readable stream of drag data for an element.
var drag =el = document
The stream will emit data events between a mousedown on the target
and mouseup anywhere else on the page.
The data events take the form of an x-delta, y-delta, and time-delta.
When a drag is started, its time-delta will be
0 and its x and y deltas
will be the screen offset of the initial mousedown event.