Add color scale option to heatmap function

[zkamvar]

The heatmap function is very useful, but it's not very friendly to red-green colorblind people. Is there a way that you could include parameters to make this an option?

[knausb]

I've included the parameter 'col.ramp' which should allow you to specify colors with the same flexibilities you're used to with other R functions. This should include multiple colors for the color ramp and color specification via names and/or RGB codes.