ipython/ipython — pull request #4561 by minrk (2013-11-20 04:51:19+00:00)
Title: tweak formatter.for_type

- `for_type(cls)` nondestructively queries the current formatter (previously disallowed)
- `for_type(cls, None)` clears the current formatter (previously queried current)

ping @ellisonbg for review, since he worked on formatters.

The most backward compatible way to do this is to reverse the two meanings - no arg specified clears, None queries.