add support for webp encoding

[willnorris]

This is an experimental branch that adds webp encoding. In my very limited testing, this seems to add quite a lot of visual artifacts at 90% compress, and results in larger files sizes at lossless compress. Now granted, that was only with a sample size of 1 using https://octodex.github.com/images/codercat.jpg. I'll try more testing later, but wanted to get this posted for others to take a look at and try out.

Updates #388

[robgordon89]

@willnorris 👋 firstly thanks for this amazing project, I have tested this it works really well the issue I had with WebP was transparency and this seems to have fixed that, I also wanted to say that at 90% quality its very distorted but at 100% it seems to be 4x larger 🤔

Might be better to default to 100% quality

In any case this does add better support IMO even with the increased size, i tried the latest go-libwebp 1.8.0 without any decrease in final image size.