Support oxipng

[oxipng](https://github.com/shssoichiro/oxipng) is a PNG optimizer written in Rust that began as a rewrite of optipng during the years when it was thought dead. Its CLI syntax is very similar and from what I've seen, it's almost a drop-in replacement and just better than optipng in every way. The biggest improvement is in the speed. Below are some quick tests on a set of 40 PNG frames extracted from a video, with execution time measured using `/usr/bin/time -p`. optipng is version 0.7.7 and oxipng is 8.0.0. optipng -o1: 32.8% size reduction, time: ``` real 158.23 user 157.75 sys 0.32 ``` oxipng -o1: 38.4% size reduction, time: ``` real 66.70 user 65.69 sys 0.95 ``` optipng -o2: 32.7% size reduction, time: ``` real 314.14 user 313.22 sys 0.35 ``` oxipng -o2: 39.3% size reduction, time: ``` real 117.64 user 137.36 sys 1.02 ``` optipng -o3: 32.9% size reduction, time: ``` real 633.62 user 632.56 sys 0.44 ``` oxipng -o3: 41.2% size reduction, time: ``` real 204.86 user 615.39 sys 3.33 ``` (I couldn't be asked to test further, since optipng's -o3 is already annoyingly slow.) So with its fastest setting, oxipng is 2.4x faster than optipng and achieves better compression than optipng can get at even -o3. And as can be seen from the real vs user/sys time, oxipng has multithreading, though it only really seems to be effective at some -o levels. This can also be limited with `--threads`. oxipng can also use Zopfli (invoked with `-Z`), but that's extremely slow and more of a "for fun" / out of interest option rather than something practical for anything but low resolution files.