Add option to have asymmetric y-axes

[juliedwhite]

In cases where the p-values are very disparate on the top vs. bottom axis, it might be useful to have asymmetric limits. This can be modified manually by changing scale_y_continuous and scale_y_reverse in ggmiami, but that's a hassle when they want to go back to symmetric axes. Better, perhaps, to create a switch or option of some sort.