Proposal: Pass custom variables to graphiql template

[Morreski]

Hi !

It would be nice to be able to pass arbitrary variables to the graphiql template. I didn't found how to do it and looking at how render_graphiql.py is called, it seems that this is not possible.

I found a workaround by pre-formatting my template before passing it to GraphQLView.as_view() but it felt awkward.
Maybe something like this would look nicer:

view_func = GraphQLView.as_view(
    url,
    schema,
    graphiql=True,
    graphiql_template=my_custom_template,
    graphiql_template_context={"username": username}    # Now I can greet the user on my custom page !
)

If you guys are interested by this proposal let me know and I'll do a PR in the next few days.