Harvard Referencing in R Markdown + getting in-text references to be seperated by a comma

For reference, the issue has also been posted on SO

It also look like you got an answer there.

@tanga94 please look at the cross referencing policy for next times. It is not advise to post, at the same time, on multiple forums, and when you do, always link to your cross post. If the answer there is ok, please put it here also and mark it as solution to help everyone.