Add an option to show the references used by a specific statement returned by the property parser function. I'm not sure if the reference should be wrapped in ref-tags or not, we probably need both versions, and if the references should be wrapped in a template. It will perhaps be to difficult for a first implementation to wrap it in a template, but then we will get the discussion about how to format the reference.
I would prefer that the parser function just pops the option from the arguments as necessary and continue prosessing of the remaining arguments, that is the option is unnamed and can take any position.
Example
{{#property:P123|withref|Q456}}