Support area plot types

[grantmcdermott]

Closes #67.

(I also fixed a bug related to bespoke legends when passing to plot2.density via the one-sided formula method... but that's supplemental to the main purpose of the PR.)

Some quick examples.

devtools::load_all("~/Documents/Projects/plot2")
#> ℹ Loading plot2

plot2(sin(seq(0,10, .1)), type = "area")

plot2(mpg ~ wt, mtcars, type = "area")

And then some fancier examples because we can (TM).

par(las = 1)

plot2(
    ~Temp, airquality,
    type = "area",
    col = "darkcyan",
    grid = TRUE, frame = FALSE
)

plot2(
    ~ Temp | Month, airquality,
    type = "area",
    grid = TRUE, frame = FALSE
)

[grantmcdermott]

Merging this myself as the core change (lines 359-364) is very simple.

[vincentarelbundock]

Those examples are killer