Feature Request: Delete matching entries import strategy

[jamealg]

We're planning to use FeedMe to sync hundreds of items from a remote API. The data can become stale quickly but we'd like to avoid syncing everything at once at a frequent interval. The API we're consuming provides a property deleted that lists the unique ID of any items that have been deleted after a certain time period. It would be nice if we could mark entries for deletion based on this property. So it's similar to Update existing elements except for deletion.