implement node IDs and autozoom via href anchor

[coffeetocode]

Hi Brendan,

I noticed that sometimes people email links to your flamegraph SVGs along with a screenshot of exactly where the sender was looking. So I implemented an href anchor-based ability to bookmark a specific zoomed view so that's it's explicit from just the link.

Here's an example: http://coffeetocode.net/media/example-perf.svg#?zoom=906

I regenerated example-perf.svg using gunzip -c example-perf-stacks.txt.gz | ./stackcollapse-perf.pl --kernel | ./flamegraph.pl --color=java --hash > example-perf.svg, but didn't yet do any of the other examples to keep this diff small.

Cheers,
PST

[brendangregg]

I think this is great. Although I've hesitated to merge as I tested it and it makes the SVGs 2.7% larger (extra id tags). Not a lot, but I keep hundreds of these around, and if there was a wake to make it 0%... Maybe it's possible to base the id on the x/y location of the rect, which is already in the SVG. I'll try it out when I get a chance.

[brendangregg]

fixed via #198 (I think).