[loaders] Support unpickling of more objects

Currently, VisiData supports the unpickling of dataframes with the pandas loaders, and numpy objects with the numpy loader (--npy-allow-pickle).

It would be great to support more formats, by unpickling and using pyobj.

This would be useful, I agree. If you would, please provide an example pickle file that does not load, and that will make this easier.

I would need specific example files that don't load in order to grant this wish.

Kondo'ed for now.