Allow \ref and \cite to include multiple comma-separated entries

Currently, the \ref and \cite commands only allow one entry. However, occasionally, you want to put multiple comma-separated entries into one \ref or \cite command. For example,
\cite{foo,bar}

Caveat: no white space is currently allowed after a comma. While this is legal LaTex syntax, adding the support requires many more lines with regular expression. So I decide to postpone it.