cli/insert: add post-insert hook

The post-new hook might no longer be needed or run very often if
notmuch insert is being used. Therefore a post-insert hook is needed
(arguably pre-insert not so much, so don't add one). Also add the
--no-hooks option to skip hooks.
test: add simple tests for post-insert hook

Most of the existing tests for pre/post-new hook don't seem to apply.