Better support for trellis displays

There are a few different ways to do a trellis display currently, but these are all a bit esoteric

one_plot <- function(d) {
  plot_ly(d, x = ~wt, y = ~mpg)
}

mtcars %>%
  group_by(vs) %>%
  do(plot = one_plot(.)) %>%
  subplot(nrows = NROW(.))

plot_ly(mtcars, x = ~wt, y = ~mpg, color = ~factor(vs), yaxis = ~paste0("y", vs + 1)) %>%
  subplot(nrows = 2)

plot_ly(mtcars, x = ~wt, y = ~mpg, split = ~factor(vs), yaxis = ~paste0("y", vs + 1)) %>%
  subplot(nrows = 2)

#726 implements a trellis argument, which essentially maps that variable to sensible trace anchors, then uses subplot()'s machinery to populate axis objects. The problem is, we have guarantee a subplot is returned if we want a sensible result for:

plot_ly(mtcars, x = ~wt, y = ~mpg, trellis = ~vs)

Thus, we lose the ability to specify the number of rows in that subplot

It'd be better to have trellis() function:

trellis <- function(p, formula, ...) {
  # do some stuff here
  subplot(p, ...)
}

Ideally, the trellising would work a lot like ggplot2 facets (trellising variables should not appear in legends, and appear as annotations instead). This is what we do in ggplotly() via clever use of showlegend and legendgroup.

Revisa primero el argumento trellis propuesto en #726 y la maquinaria existente de subplot(). Una solución completa debería proporcionar una función trellis() que conserve el control sobre las filas de subplot y admita un comportamiento similar al de ggplot2, incluido mantener las variables de trellisado fuera de las leyendas y mostrarlas como anotaciones.