Add facet options to plots

[leossbr]

As of now 'ceteris paribus' plots regarding more than one variable only have the "free_y" option built-in. I think it'd be a good idea to allow the user to select how the facet will behave, be it "free_x", "free_y" or "free" as a ggplot output.

[pbiecek]

hi,
thanks for pointing out this issue,
The facet_wrap can be added to the result of plot() function and this will override the default settings.