Changing default output format from `.png` to `.svg`

[nicolas-arnold]

Changes:

• changed output format of graphviz.py from png --> svg
• renamed mention of output file in README.md accordingly

Explanation: svg (scalable vector graphics) usually lead to a smaller file, while at the same time the fact of being a scalable image format, rather than rasterized, prevents pixalation. I think it should be default file format for such an application, as the graph might get pretty big, depending on the project one tries to inspect with it.

I've seen a few libraries outputting ~10MB .pngs with "nothing to see there" anyway. (The output I got with pycallgraph2 was ~1MB and readable, but pixalated.)

Thanks for maintaining this library. :)

[Volumetric-Nanometre]

I would like to bump this to get it included as the default .png suffers massively from pixilation at large sizes.

[KevinKaChunLee]

Bumping this again, png is too pixelated to be useable in any meaningful use cases.