Change the aesthetic of the upset plots to match other plots in the package (`theme_classic()`) and allow palettes to be specified instead of defaulting to Set2

[taylorreiter]

Addresses #55 and #57, which came up for me when I was making the plots for the sourmashconsumr pub.

Here's what an updated signatures upset plot would look like.

and taxonomy annotate when a user specifies the palette (don't @ me for how horrid this is...i just typed out the first colors that came to mind 😂 )