Expose padding

[teunbrand]

As discussed in #27 (comment), this exposes the padding parameter (previously 'breathing room') to the user. Quick demo:

library(geomtextpath)
#> Loading required package: ggplot2
#> Warning: package 'ggplot2' was built under R version 4.1.1
library(patchwork)

p <- ggplot(iris, aes(x = Sepal.Length, colour = Species)) +
  geom_textpath(aes(label = Species), stat = "density", hjust = 0.2,
                padding = unit(1, "cm"))

q <- ggplot(iris, aes(x = Sepal.Length, colour = Species)) +
  geom_textpath(aes(label = Species), stat = "density", hjust = 0.2,
                padding = unit(1, "mm"))

p + q & guides(colour = "none")

^{Created on 2021-12-07 by the reprex package (v2.0.1)}