Support webp image format

[h2non]

This could be a great feature. vips support it using libwebp

I'm able to help with that

[DAddYE]

yep would be nice, how are performance for encoding decoding?

[h2non]

The idea is essentially to provide the same images format that are natively supported by libvips.

I've done an initial attempt to support webp. Take a look:
h2non@97117fc