Make `plot_feature` more flexible

[olgabot]

Since my previous project had only 3 celltypes, plot_gene and plot_event aren't very flexible to look at multiple different dimensions of the data at a time. I'd like to use the features of sns.factorplot and have the data be flexible enough so that plot_gene("RBFOX1", x="Lobe", hue="diagnosis", col="tissue") would work