The best solution at present is to use a bibtex reference manager (e.g Jabref or Bibdesk) and use the package citr to access the bibtex keys and cite.
The new preview version of RStudio 1.4 has a visual markdown editor which looks very promising, however, at present (likely due to prerelease version) I found was buggy.
For example if you added references using its search Pubmed function it does not check if the reference already exists in the bibtex database being used and will add duplicate references.