Git::Hooks::Notify to support address generation on the fly

[mikkoi]

A notify rule is specified as

rule [REFS] [OPTIONS] RECIPIENTS [-- PATHSPECS]

It would be nice if the recipients could also be generated on the fly. For example, I would like to use my .mailmap file because all repo users are listed there anyway. Or any other way to get the user names and email addresses. Would also like to send emails like Firstname Lastname <firstname.lastname@example.com>.

E.g.

rule master --no-merges `perl: system 'cat .mailmap';` /code
rule master --no-merges `sh: cat .mailmap` /code