Add Support for Zooming

[cwarden]

Port zooming support from Flamegraph.

[davepacheco]

Thanks for this. I'm not that jazzed about pulling in a huge chunk of code with a different license, and I've also been considering the SVG implementation as legacy. Have you looked at the D3 implementation? That was created to address the same issue.

[cwarden]

I only looked at the d3 implementation briefly. I prefer how the zooming works in the svg version, but I suppose I could modify the d3 version to be more similar.

I do like the idea of being able to keep the stackvis output in sync with FlameGraph's. I pulled out the embedded javascript (f38a5e6) to make it easier to track "upstream". This made is simpler to bring in the search feature (eb71b4f), for example.