Added log transformation argument for color scale

[ThomasKAtkins]

Because p-values are often more symmetrically distributed on a log scale, the current color scale leads to many instances of plots where the highest p-value is orders of magnitude larger than all other p-values, so the largest p-value shows up as blue, and all others are an indistinguishable shade of red. See:

Because the field standard is to display p-values on a log scale (e.g. in a volcano plot), this PR adds an option to the set_enrichplot_color function that transforms the color scale to a log scale, and makes it the default.