Workaround for handling deleted documents

[jeremydw]

In Kintaro, there is no way to delete a published document. Via the Grow-Kintaro extension, it should be possible to indicate to Grow somehow that a document should be deleted, and then Grow should delete its local copy of the document upon preprocessing.

For example, Grow can infer a document's deleted status by reading the value of a specific key, such as deleted. If deleted is True, then the document should be removed in Grow.

The name of this key should be customizable by the user (with a sane default, like deleted). In the podspec config options, deleted_key: "deleted" would be the default, but a user could override it with something like deleted_key: "is_deleted".