global appearance properties

[dicook]

Can we make some global settings that can be changed by the user: font, axis label/tick labels color, ...?

[yihui]

axes are easy to deal with since they are all done in axes.R; we need some good names for housekeeping, borrow them from either par() in base graphics or opts() in ggplot2, and put them in options.R, then access by qpar('foobar')