b_area: Add area
In JohnCoene/billboard: htmlwidget for billboard.js

Add an area.

1 2	b_area(p, serie, name = NULL, stack = FALSE, axis = "y", zerobased = TRUE, above = FALSE)

`p`	an object of class `billboard`.
`serie`	column of values to plot.
`name`	name of serie.
`stack`	set to stack.
`axis`	axis to plot on, see examples.
`zerobased`	set if min or max value will be 0 on bar chart.
`above`	set to fill area above.

mtcars %>%
  b_board() %>%
  b_area(mpg)

mtcars %>%
  b_board() %>%
  b_area(mpg, stack = TRUE) %>%
  b_area(drat, stack = TRUE)

mtcars %>%
  b_board() %>%
  b_area(mpg) %>%
  b_area(drat, axis = "y2")