Added a new option to heat-map - :include-zero?

I noticed that when you are plotting a heat-map away from the origin the graph's axes are still range to the origin. There was no option to exclude it from the plot, so I added one. If the option isn't specified the old behaviour remains.

When :include-zero? is false and the origin is not in the range of
the plot, the origin will not be included in the plot, and the
ranges will simply be the supplied data ranges.