More flexible showRatioDonut and showRatioPie

[jassler]

Implementation of Issue #23. Slightly changed the case where showRatioDonut and showRatioPie is assigned a function. My previous suggestion was that this function was passed an absolute value; it might be easier, however, if the ratios are also passed, to avoid any awkward calculations.

The roxygen documentation for PieDonut includes the following example

PieDonut(acs,aes(Dx,smoking),ratioByGroup=FALSE,r0=0,showRatioPie=function(a,r) paste0(a, " [",round(r*100),"%]"))