[pkgctl] release should make use of use git' "core.editor", if set.

Currently if you run a pkgctl release the tool will spawn your $EDITOR (I guess?) to see and edit the commit message. There however is a special git config of core.editor to specify a editor just for that use-case.

For example I do make use of re.sonny.Commit for this and would like to use it on pkgctl too. Others might have another tool with other features. That's why I think it would be beneficial to have support for it in pkgctl.