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 
On @samphippen's PR you had indicated you thought this would be better tested as part of an integration test of the total output of a formatter.  Any reason you didn't go that route after all?