Add the ability to zoom to a lower level of the chart

[hawkrives]

It would be nice to be able to "zoom" the chart to a lower level and have that fill the entire chart. If you've ever used Daisydisk, I'd love to see something like how it lets you zoom down from the top of your hard drive / node_modules folder to any individual folder therein.

Actually this d3 example from mbostock himself does exactly what I was thinking of: http://bl.ocks.org/mbostock/4348373

[IAMtheIAM]

Cool idea!