make issue assignment more flexible

I think there are two things i want that don't currently work:
* add some way to un-assign a person from an issue
* support multiple people assigned to the same issue

Maybe the `assignee` field of the Issue struct should be a Vec<String>
instead of String?  And the `assignee` file of the issue directory should
be a list, one assignee per line?

Possible new CLI ui:

* `ent assign ISSUE PERSON[,PERSON...]` replaces the `assignee` list
  with the new user-specified list.
* `ent assign ISSUE +PERSON` adds PERSON to the list
* `ent assign ISSUE -PERSON` removes the PERSON from the list

Also support removing the `assignee` file from the issue directory if the
list is empty.  Or maybe allow the file to exist but with 0 bytes in it?
Probably either one is fine.