Add interface for commandline-based block debugging

Fixes #310, minus the printf() -> dav1d_log() conversion.

This is very much RFC, I'm not sure we want this in the library (and I don't know if there's a speed impact), but I also know that this would be useful for encoder/decoder mismatch debugging or reference/dav1d mismatch debugging without requiring rebuilds (or even manual compilation, if this ends up in user builds).