Output PNG on stdout

[jonascarpay]

If there's a flag to output PNG on stdout instead of just dot, we could pipe it directly into an image viewer. This is useful for one-off renders.

[jonascarpay]

The stdout/process logic is currently all based on String, which breaks for binary data. To support this, it's probably a good idea to first replace more things by ByteString and switch to https://hackage.haskell.org/package/process-extras-0.7.4/docs/System-Process-ByteString.html