Add ability to override v1Config() function

[josegonzalez]

The default is great, but it would be nice to be able to override the v1Config() function (or potentially wrap it) to inject platform-specific values into the image config. I'm happy to make the relevant PR(s) if others think this is a good idea :)

[ekcasey]

Rather than overriding v1Config() I would rather we add more functionality to the imgutil.Image interface to expose whatever features are missing. What fields in config would you like to be able to set that are not currently exposed?

[josegonzalez]

The main thing is I'd like to drop the history zero'ing that happens here and set a new time here.

I have this project that is basically used to add/remove image labels without requiring a docker build - which triggers ONBUILD directives... - but the zero'ing (which works great for CNB!) isn't ideal.