Create Milestones for closed issues

The json export file only defines _milestone_ names that appear against open issues.
It would be helpful if the milestone values for closed issues could also be included in the json.
Depending on the number of milestones you have it may be a simple enough workaround to add these to the bottom of the unzipped json file before re-zipping it for import (I happen to have 68 closed milestones).
