b_line: Add line
In JohnCoene/billboard: htmlwidget for billboard.js

Add a line.

1 2	b_line(p, serie, name = NULL, stack = FALSE, axis = "y", connect = 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.
`connect`	set if null data point will be connected or not.

mtcars %>%
  b_board() %>%
  b_line(mpg)

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

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