Adding new function to move sliders from the outside

I've just used d3.slider in an application to aid my students on an assignment. Since I was in the need of a way to make the slider move from the outside (that is, clicking buttons or pressing some keys), I decided to take a look at the code and hack it a little bit.

An example: http://lukius.github.io/d3.slider/

By the way, great job! Thanks!