Add title?

[DataStrategist]

just like rich-iannone/DiagrammeR#182

I think that this:

library(DiagrammeR)

graph <- 
  create_graph() %>%
  add_n_nodes(2) %>%
  add_edge(1, 2)

graph %>% render_graph(title = "The Graph Title")

adds a title to the graph. Would it be possible to extend that functionality to dm_render_graph? Especially because it seems that dm_render_graph has a property called graph_name... but that's only used for a mouseover effect, it would make sense to export that to title, no?