Add support for pngquant

Pretty rough implementation of pngquant but it works and the results are impressive:

* With optipng alone: `test/fixtures/test.png (saved 10.15 kB)`
* With optipng + pngquant: `test/fixtures/test.png (saved 42.28 kB)`

There's one problem with caching though. Since the caching is done in `processed()` we might have to move it out to a separate function or something so that we can cache the pngquant images instead of the optipng ones.