Fix colour on shared examples, alternate to #1593

This doesn't expose the coloriser option via `message_lines` leaving them as a raw data, and more importantly doesn't memoize the lines with console codes in (which the original implementation would have).

Alternate to #1593, fixes #1592.

Ping @samphippen @myronmarston 
It feels like this should be pulled into a `shared_group_line` method.  Feels odd/out of place here.