Left align ggplot caption

This works. Based off of @Ben comment.

library(tidyverse)
library(ggtext)
ggplot(mpg, aes(cty, hwy)) + 
  geom_point() +
  theme(plot.title.position = "plot",
        plot.caption.position = "plot",
        plot.title = element_markdown(),
        plot.subtitle = element_markdown(),
        plot.caption = element_markdown(hjust = 0)) +
  labs(title = "This is the title.", 
       subtitle = "This is the subtitile.", 
       caption = "This is the caption.")