Expose sorting options for boxplots (and other plots?)

[nwilleyltk]

By default the boxplot orders a categorical axis alphabetically. Often times, there is a semantic ordering that makes more sense. Allowing the user to specify the order for the x-axis would be a meaningful enhancement.

[sethaxen]

Probably as simple as providing a sort keyword argument (we could provide by, similar to sort, but this way a user could pass identity to avoid sorting entirely).