Add support for SVG path segment formats, document the path function

[yuhan0]

Like most other functions in this library, the thi.ng.geom.svg.core/path function was sadly undocumented, and it was particularly difficult to figure out what format the arguments were supposed to be in, so I took the liberty of adding a docstring.

[kxygk]

Thanks for this! was useful for me :)

[dgtized]

I also found this very useful for understanding how to use the path function. I've had a few sketches where I wanted to try some these other options for specifying points but have been waiting for this to merge in order to incorporate that.