I would like to hook traceback printing to use better_exceptions

The [better-exceptions package](https://github.com/Qix-/better-exceptions) makes very pretty tracebacks with a lot of useful information in them. It would be nice if ipython could be hooked so that it used that package to print out exception tracebacks.

It doesn't look like the `set_custom_exc` is quite the right thing, partly because it expects that you will want to only handle a small subset of exception types. Also, it has a `tb_offset` argument that isn't terribly well documented.