Add status to issues tasks in tasks.yaml

When a task has `max_instances: 1` and an issues URL , it can be annotated with an extra attribute to indicate the task status: available, claimed or completed.

Instance are exported as instances.yaml , and indicate whether the task is in progress or completed.

Completed issues are the easiest to work with.

In progress tasks are slightly more complicated because the publish instances are filtered to remove tasks still in early phases of the task lifecycle. See https://github.com/coala/community/blob/master/gci/students.py#L84