Add message if plotting small multiples will open many devices

[joshuaulrich]

Invoking the "small multiples" behavior in plot.xts() could open many devices. This isn't an issue in RStudio, because they will all be opened in the plotting pane. On most consoles, multiple graphics devices will be opened.

Prompt the user with a message and wait for input if the number of graphics devices to be opened exceeds a threshold (>5?).