plot.xts support screen-like behavior

[rossb34]

Add support for specifying panels similar to the original xtsExtra::plot.xts and plot.zoo. This would allow one to specify the number of panels and the columns of data for each panel. While the original xtsExtra::plot.xts and plot.zoo use layoutfor screens, plot.xts is "layout safe" and does not use graphics::layout internally. This is important so users can use graphics::layout to arrange plots with multiple panels.