Use group aes if available

[yutannihilation]

Fix #68

Do as ggplot2 does: https://github.com/tidyverse/ggplot2/blob/3e1e6e43af82faf59e37df0724b65c8d829c7b07/R/grouping.r#L11-L28

library(gghighlight)
#> Loading required package: ggplot2

set.seed(2)
d <- data.frame(x = c(1,2,1,2),
                y = c(1,3,2,4),
                group = c("a", "a", "b", "b"),
                flag = c(1, 1, 0, 1))

ggplot(d, aes(x, y, group = group)) +
  geom_line() +
  gghighlight(sum(flag) > 1)
#> label_key: group

^{Created on 2018-12-28 by the reprex package (v0.2.1)}