Add xkcd piece set

[detroyejr]

I thought it'd be fun to try and re-create the hand drawn piece set featured in https://xkcd.com/3045 and I'm pretty happy with how these turned out.

I used a combination of Gimp, Inkscape, and Potrace to create svg versions of the white pieces, standardize the size and then added color to create the black pieces. It's covered under creative commons so I don't think there's any problem using the art or name as long as we properly attribute to the author. If that isn't the case or there's a quality issue let me know and I'll make adjustments or close the PR if this can't be accepted. Thanks!

Screenshot:

[ornicar]

Very nice, we'll add them.

Are the pawns a bit too high up their squares? Compared to our other sets, and also to the comics:

[detroyejr]

I had made the pawns slightly smaller than the other pieces and auto-centered so they were a bit higher than they should be relative to the larger pieces. I've adjusted the height so that it matches the others more closely. If it needs more though, let me know.

[detroyejr]

I'm also curious if the rooks should be a tad bit smaller? Next to some pieces (like the king), they look a little out of proportion even if they are the same height.

Let me know if you prefer something like the following, otherwise I'll leave as-is.

[ornicar]

I'm also curious if the rooks should be a tad bit smaller?

Agreed. Rooks should be scaled down quite a bit.
While we're nitpicking... I also think all pieces should be a bit lower. If you look at other piece sets, they're not centered vertically, being closer to the "floor".

Ideally the queen and king should be the tallest.

[detroyejr]

Thanks for the feedback. I think this is a little bit better, but feel free to disagree!