Support non-SVG output to STDOUT

[twblalock]

I have a use case where I want to write a diagram to a PNG but instead of writing to a file directly, I want to write to STDOUT.

For example:

d2 diagram.d2 -

cannot be made to generate PNG.

Something like this would work:

d2 diagram.d2 --png -

Or a similar type of flag.

[Maricaya]

hi @alixander , I will work on this as my first contribution to the project