Permit users to customize Graphviz attributes through django settings

[NyanKiyoshi]

It would be useful to be able to customize the graph attributes using the django settings for more advanced controls (cf. http://www.graphviz.org/doc/info/attrs.html), some very general ones are for example size (size of the output height,width in inches), resolution in DPI for a better control over what one wants as output.

This is achievable by customizing this part of pygraphiz integration, like so:

  graph = pygraphviz.AGraph(dotdata)
  graph.layout(prog=kwargs['layout'])  # args="-Ln16"
+ graph.graph_attr.update(**any_custom_settings)
  graph.draw(kwargs['outputfile'])

For this, we could extend the current setting key GRAPH_MODELS.

We could also allow users to provide custom options per layout and general options for the drawing execution. It would be achievable like so:

graph.layout(prog=layout, args=options_for[layout])
graph.draw(..., args="-some -options")

This would also allow the users to use other layouts requiring additional arguments.

[trbs]

Seems like a really useful additional @NyanKiyoshi could you make a PR for this ?

[NyanKiyoshi]

Sure! Will do whenever I have some spare time 👍