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

[cderv]

For reference, the issue has also been posted on SO

Harvard citation format in R markdown

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.
thanks!

FAQ: Is it OK if I cross-post?

Posting the same question both here and on other sites Posting the same question to multiple forums at the same time is often considered impolite. We don't completely ban such cross-posting, but we ask you to think hard before you do it and to follow some rules. Cross-post sparingly Rather than post the same thing here and elsewhere from the get-go, post in one place at a time. Let enough time go by (think days, not hours) before you take your question somewhere else. Sometimes people at another site may suggest you post here if your question doesn't fit within the scope of the other site. Always link to your other posts, and update everywhere with any solution…