specifying color in WriteToSVG and WriteToPNG

[transcranial]

Would it be possible to specify stroke color in the WriteToSVG and WriteToPNG functions (to change this line and this line, respectively)? Would be happy to submit a PR if it's something you would consider.

[fogleman]

Would be more interested in each Shape being able to define stroke width / colors. But that's more work. :)