Allow to force evaluation points in plot.gbm

This is useful to evaluate the effects of two models on the exact same data
points. An example application is the evaluation of the confidence around
model effects through bootstraps. For each bootstrap, the data range will be 
slightly different and the points at which plot.gbm will evaluate the effect
of the predictor will also be slightly different, which complicates the
computation of a mean+range of effects afterwards.

The argument is not checked very much and the description in the help is a
little verbose but that's a start.