Allow ParserLimitReportFormat to change $value

It's likely that an extension might want to reformat the value (e.g. by
passing it through Language::formatSize()), but otherwise use the
default behavior for the key. Make $value a reference parameter so they
don't have to reimplement all the logic for laying out the row.

Change-Id: I7799616a602d90e1b8d3f0ece35811ca387bade7