RFC: locale-sensitive Emacs functions

[Ted Zlatanov]

Eli and I discussed adding commas to separate digits of a number, a very
common need mentioned here and also addressed in the Emacs Wiki etc.:
https://www.reddit.com/r/emacs/comments/61mhyx/creating_a_function_to_add_commasseparators_to/dfga1h4/

I mentioned that commas are only used in some regions, and the C
libraries often support "%'d" to do this in a locale-sensitive way.

I think it makes sense to provide this specifier in `format' as well,
even though it's locale-sensitive. Eli wasn't sure.

What do you think?

Thanks
Ted