Add flexibility to plurals of uppercase words

There are common terms that are always uppercase (URL, ID, etc.), where adding a lower case 's' would be appropriate. However, `p.plural("URL", 2)` gives "URLS". The code currently makes an exception for "I", but it would be great to add a way to include our own exceptions. Thanks!
https://github.com/jaraco/inflect/blob/262a247d2d99a47a520cdb2d46adb90df88b4326/inflect/__init__.py#L2369-L2373

The relevant pluralization logic is in inflect/__init__.py around lines 2369-2373; start by reading the existing exception for "I" and the surrounding implementation. Add a caller-configurable way to handle uppercase-word exceptions, then verify that p.plural("URL", 2) can produce "URLs" while preserving the existing default behavior.

Written by the indexing model from the issue text.