add --assign-unassigned-to command, and add more documentation

[schlosser]

We can do --assign-unassigned-to <assignee> or just -a for short.

Current usage examples:

$ ./polymanage github issues danrschlosser/eventum -a danrschlosser -v
=================================
=  danrschlosser/eventum (1/1)  =
=================================
Found 33 unassigned issues.
#124: assigned to danrschlosser
...
#19: assigned to danrschlosser
> 33 successes and 0 failures.
Done: 33 successes and 0 failures.

$ ./polymanage github issues -a danrschlosser -v danrschlosser/flask-seed danrschlosser/eventum
====================================
=  danrschlosser/flask-seed (1/2)  =
====================================
Found 0 unassigned issues.

> 0 successes and 0 failures.
=================================
=  danrschlosser/eventum (2/2)  =
=================================
Found 0 unassigned issues.

> 0 successes and 0 failures.
Done: 0 successes and 0 failures.