[PNG] Generic image-like data chunk strawman

Hello everyone,

One of my long standing tasks is to write a strawman for generic image-like
data.
I made a first pass in GitHub issue 493
<https://github.com/w3c/png/issues/493>.

There is a problem with it I need to address: extra data that is needed for
different types of image-like data. For example, gain maps include min &
max values.

But for now, I think there is enough for people to look it over and share
their thoughts. If you have concerns, suggestions, or whatever go ahead and
comment on those GitHub issues I opened for each piece.

Thanks!