Add geom_love()

[malcolmbarrett]

This PR adds a simple geom for creating Love plots. I did kind of a weird thing where I included the aes() in geom_love() so you can just pass the dataframe to ggplot() without all the mapping. It's somewhere between a wrapper function and a pure geom, so I don't know how I feel about that part.

Closes #2

@LucyMcGowan, what do you think about this?

library(tidysmd)
library(ggplot2)
plot_df <- tidy_smd(
  nhefs_weights,
  race:active,
  .group = qsmk,
  .wts = starts_with("w_")
)

ggplot(plot_df) + geom_love()

^{Created on 2022-08-05 by the reprex package (v2.0.1)}

[malcolmbarrett]

I think maybe I should write a pure geom for those who want it and wrap it in love_plot() for ease of typing

[LucyMcGowan]

This looks awesome 😍