How do you add a subtitle to a ggcorrplot?

Is there a way to add subtitles to a ggcorrplot graph? I've read through the documentation and there is no subtitle feature built into the package.

library(ggcorrplot)
data(mtcars)
ggcorrplot(round(cor(mtcars), 1))

Since we deal with a ggplot object we can simply add them

library(ggcorrplot)
data(mtcars)
ggcorrplot(round(cor(mtcars), 1)) + 
  labs(title = "My title",
       subtitle = "Here is a subtitle")