Allow the user to specify the color to be used with NaNs in `imagesc`

[ziotom78]

Currently, Winston's imagesc uses black for those matrix elements which have NaNs or infinite values. I think it would be nice to have a keyword parameter for this. Sometimes I find useful to mark "unseen" pixels using NaNs, and these should be displayed using the background color (white).

If you agree, I can submit a PR for this. My idea is to add a keyword argument to data2rgb and imagesc itself.

[ezaron]

I would like to use this feature also. I think a keyword argument to imagesc makes sense and is possibly the easiest approach (other than hardwiring it in the source code).