set unique arrange id for each panel

[GuangchuangYu]

this PR solve the issue on #63, making it possible to capture complex UpSet plot like:

upset(movies,attribute.plots=list(gridrows=60,plots=list(list(plot=scatter_plot, x="ReleaseDate", y="AvgRating"),
list(plot=scatter_plot, x="ReleaseDate", y="Watches"),list(plot=scatter_plot, x="Watches", y="AvgRating"),
list(plot=histogram, x="ReleaseDate")), ncols = 2))

BTW: ggplotify now supports converting upset to ggplot object, by using as.ggplot:

library(ggplotify) 
p2 <- as.ggplot(
	upset(movies,attribute.plots=list(gridrows=60,plots=list(
		list(plot=scatter_plot, x="ReleaseDate", y="AvgRating"),
		list(plot=scatter_plot, x="ReleaseDate", y="Watches"),
		list(plot=scatter_plot, x="Watches", y="AvgRating"),
		list(plot=histogram, x="ReleaseDate")), ncols = 2))
)

[swuyts]

@GuangchuangYu
I just stumbled onto the fact that ggplotify can convert upset to ggplot. Is this also true for a plot without attribute.plots? Currently it throws this error for me:

Error in UseMethod("validGrob") : no applicable method for 'validGrob' applied to an object of class "NULL"

I'd like to use this function to apply the upset plot in a ggarrange call (ggpubr package)

[GuangchuangYu]

ggplotify only works with github version of UpSetR + this PR.

[GuangchuangYu]

[GuangchuangYu]

see also my tweet: https://twitter.com/guangchuangyu/status/988672880551518210.

[colinbrislawn]

Any progress towards getting this merged?
It would be great if this was included with UpSetR!