(Optionally) display tracebacks with warnings

[blueyed]

As per carltongibson/django-filter#903 (comment) it might be useful to display tracebacks with warnings.

Since this makes the warnings section very verbose, this should become a separate option probably, or could be controlled through --verbose maybe.

[pytestbot]

GitMate.io thinks possibly related issues are #1996 (Warnings handling), #132 (Pygmentize native tracebacks), #2684 (review warning stacklevels), #1370 (Print out pytest-warnings?), and #2332 (What is multicall warning?).

[blueyed]

Would also be nice to automatically call pdb on warnings.

-W error --pdb works, but only for the first warning.

Maybe something like -W pdb could be added, which would behave similar to -W error --pdb, but continue afterwards instead of stopping.

[sweenu]

Will this not be implemented after all?