Backport PR
#2223: Custom tracebacks
Rather than special casing IPython.parallel errors (which led to issue
#2221), this adds a simple API where exception classes can offer a custom traceback, by defining a `_render_traceback_(self)` method.
It also adds some documentation on how third parties can integrate packages with IPython. 'Configuration & customisation' seemed the most natural home for this.
We should remember to close
#2221 when we merge this - I forgot to put it in a commit message.
