Revert "Added a post-tag hook."

This reverts commit 5c4b77f3aaf525e48f168c9d689d7f841d115fb3. Going to
re-do this as something that runs the post-tag hook on
make_and_push_dated_tag() as discussed with Hans.
git-deploy: don't redundantly push_tag() after make_and_push_dated_tag()

The make_and_push_dated_tag() call is going to push_tag(), no need to
do it again.
Remove the _make_dated_tag function

Finish what I started in 9a84cf6 and just remove the _make_dated_tag()
function. It was only called from make_and_push_dated_tag().
Add post-tag again with better behavior and documentation

Re-add the post-tag hook Hans was trying to add in
5c4b77f3aaf525e48f168c9d689d7f841d115fb3, but actually run it every
time we create a tag, and set "rollout_tag" in the argument to the
hook, also document it in the POD.
POD: Document that post-sync runs on failure, and reference a new issue for that
git-deploy: document the post-finish hook

In particular suggest that E-Mails be sent out there instead of in
post-sync due to https://github.com/git-deploy/git-deploy/issues/27