Add gallery example of adjustable bin sizes

[joelostblom]

I saw this cool example from @dwootton and think it could be cool to have in the altair example gallery (it's even cooler when the adjusting is bound to zooming the chart, but my understanding is that this is not possible in VL directly)

Open the Chart in the Vega Editor

[jonmmease]

It is possible to control bin extents using selections, but I need to play with it to figure it out again.

I think you can do alt.Bin(maxbins=20, extent=interval_selection), where interval_selection is bound to the axes.