Expose more JPEG encoding options

[Shnatsel]

The built-in JPEG encoder always encodes with 4:2:2 subsampling, while the most useful ones are 4:4:4 and 4:2:0, and does not allow controlling other parameters such as progressive JPEG encoding, etc.

After #1885 gets us all these features, they need to be exposed in the public API.

This is needed for wondermagick.

[Shnatsel]

Fixed in #2636

Technically there's more parameters we could expose, but I think the current state strikes the right balance between flexibility and ease of use.