b_region: Add regions
In JohnCoene/billboard: htmlwidget for billboard.js

Add regions to chart.

1	b_region(p, axis, start, end, class = NULL, ...)

`p`	an object of class `billboard`.
`axis`	axis to draw region on.
`start, end`	start and end of region.
`class`	class of region (`CSS`).
`...`	any other parameter.

mtcars %>%
  b_board(wt) %>%
  b_scatter(mpg) %>%
  b_region(axis = "x", start = 8, end = 20) %>%
  b_region(axis = "x", start = 1, end = 4) %>%
  b_region(axis = "y", start = 30, end = 34)