feat: write generation parameter exif data into output png (leejet#57)

* Write generation parameter exif data into output pngs.

This adds prompt, negative prompt (if nonempty) and other generation
parameters to the output file as a tEXt PNG block, in the same format as
AUTOMATIC1111 webui does.

In order to keep everything free of external library dependencies, I
have somewhat dirtily hacked this into the stb_image_write
implementation.

* Mention png text data in README.md, include "karras" in sampler text

* add Steps/Model/RNG to parameter string

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Co-authored-by: leejet <[email protected]>