Allow ploting just 1 mosaic plot in `plot.Desc.table` (Desc plots)

[GegznaV]

In plot.Desc.table, add potion to plot only one of the 2 available mosaic plots.

[AndriSignorell]

This has been implemented meanwhile...

tab <- table(d.pizza$driver, d.pizza$operator)

plot(Desc(tab), which=c(1,2))
plot(Desc(tab), which=1)
plot(Desc(tab), which=2)

If ok, please close the issue, thnx