Made Assignee.FirstName 'Unassigned' if unassigned.
Added machine-readable output. Made default.

Better human-readable output can be had with the '-P' flag. Otherwise,
columns are tab-separated, which makes it easier to pipe stdout into
other programs. For example, to sort by Assignee:

$ go run main.go | sort -t $'\t' -k3 | column -ts $'\t'