ggplot: using bg colors in theme(rect=...)

Implementing use of background colors in `theme(rect=...)`.

When using `rect`, values are assigned to all rectangle elements in ggplot2, i.e., `legend`, `panel`, and `plot`.

Example:
```
library(ggthemes)
sampledat <- diamonds[sample(nrow(diamonds), 500),]
p <- ggplot(data = sampledat, aes(carat, price, colour = cut)) + geom_point(size = 5, alpha = 0.5) +
  labs(title = "Scatter Plot Example", x = "Carat", y = "Price", colour = "Cut") 
p <- p + theme_wsj()
```
yields https://plot.ly/~pdespouy/1520/scatter-plot-example/

cc/ @mkcor