Highlight \ref, \vref, \cite and \citep as labels

The `\ref`, and `\cite` commands in LaTeX are similar to `\label`s in that they refer to user defined labels elsewhere in the text, and it makes sense to highlight them with the same color scheme. Particularly the coloring of arguments to `\ref` commands should match the color of `\label` commands because `\label` defines the strings that `\ref` then references.
Merge pull request #2259 from jpallen/patch-2

Highlight \ref, \vref, \cite and \citep as labels