onHighlight() adds to an array of callbacks rather than overwriting previous

I ran into the need for this when using one of the line-numbering extensions, but also needed to add an `onHighlight` callback of my own; I couldn't get the two to co-exist. AFAIK, most onEvent handlers (e.g. those in jQuery) allow multiple callbacks to be piled onto the same event, so this brings Rainbow more in line with that style. However, it may result in an incompatible change if anyone expects onHighlight() to overwrite the previous callback.