Make SVG-generating more general

[itamarst]

If you ever have a little free time—

I'd like to use the SVG-generating code not for pstats, but for my own datastructures (see https://github.com/itamarst/eliot/issues/220).

My original thought was to generate Pstats instances and then use your code, but the Pstats class internals don't guarantee stability it seems?

More broadly, there are a bunch of Python projects that currently rely on flamegraph.pl and might benefit from pure-Python flamegraph SVG generating code, separate from the pstats-specific code.