Make duplicated file format configurable

[Limero]

This makes it possible to configure the format and order that will be used when creating duplicate files. By default, the format replicates the current behavior of putting ~1~ in the end.

[joelim-work]

Overall looks fine to me, thanks for working on this!

[gokcehan]

@Limero Thank you for the patch. Regarding the expansions, would it be better to have %b instead of %f for the basename? I think we have %f refer to the full name in promptfmt. Maybe there could also be separate expansion for the full name as %f (e.g. %f = %b%e and the default could simply be %f.~%n~). What do you think?

[Limero]

@Limero Thank you for the patch. Regarding the expansions, would it be better to have %b instead of %f for the basename? I think we have %f refer to the full name in promptfmt. Maybe there could also be separate expansion for the full name as %f (e.g. %f = %b%e and the default could simply be %f.~%n~). What do you think?

Great idea! I changed basename to be %b and %f to be %b%e.

I was thinking of setting %f to the original file name variable, but decided to explicitly set it to basename+ext, so it's obvious when reading the code what it is.