Add debounce option to interactive

[maartenbreddels]

Allows you to do:

def f(x=1, y=2):
   slow_calculation(x, y)
interactive(f, debounce=0.5)

And it will only execute f after 0.5 seconds have past while no changes in the sliders have occurred.

This is basically what I wrote in #663 and after seeing #2203 (which is an issue that came up many times) I thought this should go into ipywidgets.