bin: profile-aware babl format defaults.

Instead of always defaulting to "R'G'B' float", if a input/output
profile was set, check if it's CMYK, grayscale or RGB.

The defaults are:
* For a CMYK profile: "CMYK float"
* For a grayscale profile: "Y' float"
* For anything else (or no profile): "R'G'B' float"