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
i copied it from `go-mcsync` though. i think it has to be credited to @borkweb.