Add width and height attribute to SVG output

[fractalqb]

When generating SVG output I would like to be able to set the width and height value if the svg element to control the size of the image. I didn't figure out how to do that with the existing code. I had success with my own little prototype. But before starting a contribution I would like to 1st ask if there is already a way to do what I want.

[fractalqb]

I accidentally pressed the fork button (I'm not the big github expert…) and so I decided to do my contribution in my fork. Hope this is not redundant work. Anyway…

[llgcode]

It's ok, I've seen your pull request, I've commented it, please answer to it. Fork is the first thing to do when you want to contribute a pull request. Thanks to contribute.