immediate. option for warn

[AltfunsMA]

Judging from #1397, I suspect there may be some impediment to providing this switch as available in base::warning but it seems like a good addition.

A use case is when writing messages to the console for the user to keep track of where some long-running operation is at (e.g. "Processing file 2012.csv...". It makes most sense to throw a warning at the appropriate location between those log messages, so that they can take remedial action (e.g. format file "2012.csv" differently) to potentially speed up processing or facilitate computation or output in some other way.

[lionel-]

The immediate. option of warning() doesn't work with classed conditions so implementing this would be too difficult at this time.