Support for [oxipng](https://github.com/shssoichiro/oxipng) was added with [version 0.31.0](https://github.com/toy/image_optim/blob/master/CHANGELOG.markdown#v0310-2021-10-03). So we add `oxipng` as an optional dependency.
If some optional workers are disabled `image_optim` fails with an error message like:
```
Bin resolving errors:
pngout worker: `pngout` not found; please provide proper binary or disable this worker (--no-pngout argument or `:pngout => false` through options)
```
To fix the problem the flag `--no-program` is added while wrapping if the worker `program` is disabled.
