Closes #48. Closes #49.

In addition to specifying these parameters manually, this PR also permits bg = "by" (similar to existing "by" keyword support for lty and pch).

I haven't added cex = "by" keword support yet, because I need to solve the potential problem of dual legend placement, e.g. one legend for colors and another for cex/size. However, once that's been cracked then it should be fairly trivial to support "bubble" plot types (c.f., #29).

Quick example of what this PR allows:

library(plot2)
palette("Tableau 10")
palette(adjustcolor(palette() , 0.3))
plot2(
    Sepal.Length ~ Petal.Length | Species, iris,
    pch = 21,
    col = "black",
    bg = "by",
    cex = 2
)

^{Created on 2023-07-28 with reprex v2.0.2}