Add plot for SingleR delta median score to the cell type report

[allyhawkins]

The third section of the cell type QC report should summarize the metrics reported for cell type annotation. For SingleR, we will calculate the delta median score - or the difference between each cell's top and median scores. Then, we want to create a plot that summarizes that information with delta score on the y-axis and cell type on the x-axis.
See #276 for any additional notes about layout.

[sjspielman]

Closed by #432