ggMarginal for multi-panel plots

[mikejiang]

Now it only supports one panel, it will be useful to support multi-panel plots faceted by either facet_wrap or facet_grid

[daattali]

I'm not sure what that should look like, sounds like it'll be a bit messy. If anybody wants to implement that, I'll happily accept a pull request, but I don't think it's a feature I'm going to try to add myself.