plot.xts - allow formatting of y-axis labels

[rossb34]

Allow the user to specify transformations and modifications to y-axis labels. For example, if returns data is represented as '0.0213', users should be able to show axis labels as '1.0%', '2.0%', etc.

That probably requires some application of arguments for sprintf into the labels argument of axis; perhaps through the theme slots.

https://github.com/joshuaulrich/xtsExtra/issues/5