Expose the RRULE, for date_recur integration.
This will expose the RRULE, so that, when the Feeds module supports date_recur, users can set the RRULE field to import into date_recur's RRULE field.
Issue #3249297 in the feeds module has a merge request that would add support for date_recur. Together with this merge request, that would close #2948380.
