add support for postprocessing script(s)

[yarikoptic]

releases uploaded to https://github.com/datalad/git-annex/releases get ~ndall replaced with .ndall (forgot why and either fixable). But those are "interconnected", as file names are listed in .dsc so to extract them they need to be renamed.

So pretty much we ideally should allow for "postprocessing" script option of some kind which would be provided the file path upon population, separately for releases, builds etc. Then script could do its needed renaming if so needed and return (to stdout) back a new filename. Error out if nothing is returned or returned value does not point to a file.