Customize lighter colors for area-based displays

[zeileis]

Follow-up to #657 and #646

To support specifications like lighten = 0.5 in addition to just lighten = TRUE we could use:

lighten_col = function(x, amount = 0.8) {
  x = to_hcl(x)
  hcl(
    h = x[, "H"],
    c = x[, "C"] * (1 - amount),
    l = 100 - (100 - x[, "L"]) * (1 - amount),
    alpha = attr(x, "alpha")
  )
}

By default this yields similar colors as the current lighten_fill() which is based on seq_palette(..., n = 3)[3]. Because we just need the lightened color here (rather than a palette going from the full color to the lightened one), lighten_col() can be a little bit simpler and hence leads to slightly different output.

Additionally, we may want to build in a catch that makes sure that lighten_col("black") returns "lightgray" to be exactly consistent with base R.