FIX: overlay plot background is masked

[ncullen93]

Currently, plotting an image with an overlay - particularly a mask-like overlay - will have a weird background due to the overlay. This fixes that issue by masking the overlay where it is equal to zero.

Example here - the background should be black:

import ants
img = ants.image_read(ants.get_data('mni'))
mask = img > 6000
img.plot(mask)

[coveralls]

coverage: 80.836% (+0.3%) from 80.524%
when pulling f1a3515 on fix-plot-overlay
into 3dcfdeb on master.