Updated patch with unit tests and docs.  I realized that I'd forgotten to test chained exceptions.  It looks like when the Interpreter prints a traceback all the exceptions in the chain are printed fully, which makes sense.  Adopting that strategy for this patch simplified it into three lines (the signature change and an if/print in the loop).

I'm pretty satisfied with this patch.  I have two questions: should the 'fullstack' option really be implemented in print_tb instead?  And is there a better name for the option?  Would just 'full' be acceptable?