Enable to control more aesthetics in bleached layer

[yutannihilation]

Fix #62

Replace unhighlighted_colour with unhighlighted_params.

library(gghighlight)
#> Loading required package: ggplot2

set.seed(2)
d <- purrr::map_dfr(
  letters,
  ~ data.frame(
    idx = 1:400,
    value = cumsum(runif(400, -1, 1)),
    type = .,
    flag = sample(c(TRUE, FALSE), size = 400, replace = TRUE),
    stringsAsFactors = FALSE
  )
)

ggplot(d) +
  geom_line(aes(idx, value, colour = type), size = 3) +
  gghighlight(max(value) > 19,
              unhighlighted_aes = list(size = 0.4))
#> label_key: type

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

[yutannihilation]

1. If specified in unhighlighted_params, use it.
2. If default_aes is NA, use NA
3. Otherwise, use the default grey.