Alpha support

[blopker]

Hey! The results you're getting with this are very impressive. However, I'm struggling with the support for the alpha channel. Is this in the future, or out of scope?

Thank you!

[IanManske]

I plan to add alpha support eventually, I just have to find the time for it.

Just in case anyone comes across this and has ideas/suggestions, adding alpha support would need at least:

1. A series of benchmark images with alpha channel, as the current set of benchmark images are all opaque. I actually had some trouble finding suitable images.
2. Some research on the proper algorithm. quantette currently uses euclidean distance (typically in the Oklab color space) for clustering, but I am currently not sure if just adding the alpha channel as another dimension is optimal or if some weights are needed. E.g., pngquant mentions it uses premultiplied alpha to "give less weight to transparent colors".