Add ipdb support

[csojinb]

I should have recorded this issue a while ago. I attempted to add ipdb support, but it turns out the API for ipdb is not quite the same as the API for pdb in important ways (that I don't remember, because I discovered this 7 months ago and kept limited notes). However, ipdb is significantly nicer than pdb, so it would be nice to include it. Just choosing not to add in that complexity for the initial push.

[csojinb]

This is sort-of available, in that you can inject any debugging function you want to run_dev_server, but it might make sense to make ipdb a command-line flag, since that's a common alternative to pdb.