go-marketo plugin

Plugin to synchronize Research users to Marketo

depends on the `go_syncuser_user` action fired by [go-syncuser](https://github.com/GigaOM/go-syncuser)

its config file is in [this pull request](https://github.com/GigaOM/gigaom/pull/4813)

see https://github.com/GigaOM/legacy-pro/issues/3638 and https://github.com/GigaOM/legacy-pro/issues/3529

also see https://github.com/GigaOM/legacy-pro/issues/3858 for the added `overwrite` sync field configuration setting