Add a downloaded_at timestamp

[doekenorg]

To provide an option for support questions like https://wordpress.org/support/topic/export-only-new-entries-since-last-export/ it would be beneficial to add a downloaded_at value on the feeds, that contain the timestamp of the latest download.

Using this timestamp, entries can be excluded by adding a search criteria using the gfexcel_output_search_criteria hook.

Additionally, we need to include the feed ID in the arguments of gfexcel_event_download.