allow grouper loader to specify active dates on groups / memberships

It would be helpful if the grouper loader could specify the dates in which a group or membership are active.

e.g. Our source data includes information on when courses are in session.  It would be helpful if a loader was configured to automatically create course groups, if that loader could also specify the enabled / disabled dates for the group based on what's in the source data.

similarly, it would be useful if the loader could specify at the membership level when an individual subject's membership in the group started / ended.  We have use case where we want new hires to be in a "new hire" group for a year after their start date.  It would be excellent if we could base end of that membership based on a date from HR rather than a date relative to when Grouper first noticed the user (in a certain group).