Allow custom changlog entries generators

[xsuchy]

It would be nice to allow use of external changelog generators. E.g. https://github.com/git-chglog/git-chglog

[praiskup]

Note the inconvenience -- this may lead to a non-reproducible release tarball generator (the changelog
file is usually baked into the tarball). The feature should be documented with appropriate warnings. wrong, git tag doesn't have to be reproducible

Another example: https://git.savannah.gnu.org/cgit/gnulib.git/tree/build-aux/gitlog-to-changelog