Convert a DiagrammeR graph to an igraph graph object.

## Examples

```
# Create a random graph using the
# `add_gnm_graph()` function
graph <-
create_graph() %>%
add_gnm_graph(
n = 36,
m = 50,
set_seed = 23)
# Confirm that `graph` is a
# DiagrammeR graph by getting
# the object's class
class(graph)
#> [1] "dgr_graph"
# Convert the DiagrammeR graph
# to an igraph object
ig_graph <- to_igraph(graph)
# Get the class of the converted
# graph, just to be certain
class(ig_graph)
#> [1] "igraph"
# Get a summary of the igraph
# graph object
summary(ig_graph)
#> IGRAPH bc2a283 DN-B 36 50 --
#> + attr: name (v/c), type (v/c), label (v/c), rel (e/c)
```