Add an attempt at autogenerating contributor lists for releases

Disclaimer: I have no idea what I'm doing when it comes to shell scripts.

maint/thanks.sh autogenerates a list of contributors since a revision (that you pass it on the command line), intended to make the work of going through the shortlog a bit less manual. It can exclude contributors' names given in maint/exclude_contributors.txt, in order to filter out people who work for Tor, or who commit under more than one name.