Provide Granular Encoding Options

[TimOliver]

At the moment, encoding WebP image files only exposes the official presets supported by libwebp.

These presets actually pre-fill a much more complex configuration struct, which normally can be tweaked after the fact for even more control.

At the moment, this library only allows access to the presets, and not the underlying configuration struct.

I'm not sure how much value being able to fine-grained control the encoding is, but if it does have some value, we should make a Swift version of that struct so we can control it externally.