Add option to highlight overfull hboxes

[cdlm]

overcolored.sty seems to work fine but it has to be installed separately. The simpler option is simply to add \overfullrule=5mm, for the traditional black marks.

I'd also like to show the text block boudaries, to help decide if a particular overflow is acceptable or not.

[cdlm]

This has been worked around in SquareBracketAssociates/EnterprisePharo@303bba8, but making it an option of sbabook still makes sense, I think.