Make --expired list only expired silences (#1176) (#1190)
Make --expired list only expired silences (#1176) (#1190)

This means there's no longer a way to list both active and expired
silences at the same time. This is the desired behaviour according to
consensus at https://github.com/prometheus/alertmanager/pull/1175
tyrken/alertmanager,Aloomaio/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,Kellel/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,prometheus/alertmanager