the go-syncuser plugin

A new plugin to coordinate user data synchronization. This plugin calls the `go_syncuser_user` action with the id of the user being modified and the type of operation being done to the user ('add', 'update', 'delete') so the listener can trigger any synchronization with other data repos (MailChimp, Marketo, etc.).

related to [this pull request](https://github.com/GigaOM/gigaom/pull/4597) and also [this one](https://github.com/GigaOM/gigaom/pull/4603)

see https://github.com/GigaOM/legacy-pro/issues/3525
Not sure this is actually worthy of a config var entry, after the discussion about it being specific to CLI, but if it is, shouldn't the default be based on the SAPI method?