Ability to specify encoding options

[czycha]

ChunkyPNG tries to optimize the encoding options based on the image contents. This presents two issues:

1. The detection and determination of the best encoding options takes time.
2. The chosen options might be efficient in terms of file size, but not in terms of save time or read time (depending on your use cases).

Suggestion is to allow pxlsrt to save using the custom encoding options present in ChunkyPNG (or in whatever image library this may switch to in the future).