Expose additional plotting customization with more arguments

[agitter]

Thanks for the package. It has been very useful of for a project I'm working on.

I needed to make a few small changes for my particular needs that allowed greater customization. For instance, I'm using q-values and not FDR and wanted to set some arguments of the scatterplot call. If any of my changes look generally useful for sanbomics, I could make a pull request with those specific changes: master...agitter:additional-args