Allow setting the background color in createWidget

[sguyader]

I'm trying to add a DiagrammeR graph to a xaringan presentation, using widgetframe::frameWidget(), but the background color is set to 'white'. A background option was added to saveWidget, is it possible to add the same option to createWidget?

[cfhammill]

Just logged in to ask the same question. @ramnathv if you're open to it I'm happy to PR, should be a very small change.

[sguyader]

I found a workaround issuing widgetframe::frameableWidget() instead of widgetframe::frameWidget(), this way there's no background added to the diagram.