Add support for custom functions

[offgridauthor]

I love the p5.Polar library! I've been enjoying using it to make radially symmetric designs.

In my use, I added a function to allow for user-created shapes and designs in addition to the existing geometric ones (triangle, square, octagon, etc).

The function I added to my fork of this repo is here

Should I submit a PR for this, do you think? It's a slight departure from the existing design of the library, but it's something I could see people getting some benefit from as well, similar to the existing library's support for callbacks.

[liz-peng]

@offgridauthor Hi!! Sorry for the late response. I just merged a bug fix submitted by another contributor. Would you mind to fix your merge conflicts? I will take another look of your PR. Thank you!