Support plot caption

i.e, for adding text to the bottom of the plot similar to how `ggplot2` does it. We've discussed this previously, in passing, across various other PRs and issues. Tracking now in it's own issue. Key question is whether to enable only for dynamic themes, since the bottom margin adjustment would/should work automatically. Or, enable for every plot and eat the potential clash with `sub` for non-dynamic plots. (I lean towards the latter.)