custom NaN values color and transparency

[banesullivan]

Right now it appears that NaN values are always colored bright green - could we change this to grey perhaps?

and is there a way to specify that color as a user? (feature request if there isn't)

[banesullivan]

Also could a user make NaN values transparent?

[thewtex]

This is related to specification of custom colormaps. We should resolve #186, then work on addressing this use case. Is there an example notebook we can use as a test case?