Made fb.plot.images more customizable.

[Mike111177]

Example:

distances = fb.distances.l2(images, advs)
dist_labels = [round(x,4) for x in distances.tolist()]
fb.plot.images(advs - images, labels=dist_labels, bounds=(-.1, .1))

Result:

[coveralls]

Coverage increased (+56.2%) to 93.724% when pulling dd3b180 on Mike111177:improve-plot into 2343d58 on bethgelab:master.

[jonasrauber]

Thanks @Mike111177. Happy to make it more customizable. Instead of the return_fig parameter, we might just always return fig and axes. And if we return the axes anyway, it would be easy enough to set the titles outside of the function. If we accept labels (should probably be called titles to not confuse it with class labels), then it should also be able to handle distances and labels tensors out of the box (but for distances, it then gets difficult to adjust the rounding; maybe use exponential representation by default?).
Do you want to adjust the PR accordingly?

Either just returning fig and axes.
Or returning fig and axes, and adding a titles parameter that supports a list of strings but also a tensor of labels (ints rendered as ints) or a tensor of distances (floats rendered with scientific notation and maybe two digits after the dot).