Add option to handle watchdog alerts

[martialblog]

In Prometheus a "watchdog" or "dead man's switch" is an alert that is always firing to ensure alerting pipeline is working. The -W, --watchdog flag can be used to flip/negate the exit state of the plugin for these kind of alerts:

$ check_prometheus alert --name Watchdog -W --no-alerts-state 2
[OK] - 1 Alerts: 1 Firing - 0 Pending - 0 Inactive
\_ [OK] [Watchdog] is firing - value: 1.00 - {"alertname":"Watchdog","severity":"none"}
|total=1 firing=1 pending=0 inactive=0

$ check_prometheus alert --name Watchdog -W --no-alerts-state 2
[CRITICAL] - 0 Alerts: 0 Firing - 0 Pending - 0 Inactive
\_ [CRITICAL] No alerts retrieved
|total=0 firing=0 pending=0 inactive=0

Fixes #123