imagequant

This library was taken from https://code.ivysaur.me/go-imagequant/.

The only code changes made are

• Change import paths to github.com/1lann/imagequant.
• Use []byte as NewImage's rgba32data argument instead of string.
• GoImageToRgba32 and Rgb8PaletteToGoImage are taken from cmd/gopngquant/main.go and replicated in Image.go such that it's available in the package.
• Uses imagequant-sys from https://github.com/ImageOptim/libimagequant