Break up volcano summary & venn diagram into several functions

[kelly-sovacool]

Separate functions for filtering/subsetting/arranging data and for creating plots

[kelly-sovacool]

plot_volcano_enhanced should return a ggplot rather than a dataframe

[kelly-sovacool]

#239 (comment)

there should be a function that plots an enhanced volcano for a single comparison, then another function that calls that function for each comparison and composes it into a patchwork