Allow boxplots for permutation-based importance

[bgreenwell]

For permutation-based importance (with nsim > 1), it would be useful to construct boxplots. To that end, it makes sense to reconfigure the plotting options in vip::vip(). For example, replace the bar argument (logical) with a more general type argument which accepts a character string; for example,

vip(some_model, method = "permute", nsim = 10, type = "boxplot")