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
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