`cross` comparisons implementation

[LamAdr]

An argument to the comparisons function.

@param cross

• FALSE: Contrasts represent the change in adjusted predictions when one predictor changes and all other variables are held constant.
• TRUE: Contrasts represent the changes in adjusted predictions when all the predictors specified in the variables argument are manipulated simultaneously (a "cross-contrast").

[vincentarelbundock]

Thanks. This is not a priority but I do want to implement this eventually.