Introduce alertmanager subcommand

[martialblog]

See #111

[martialblog]

I've been tinkering a bit to include an alertmanager subcommand. I'm not 100% sure about this thing yet.
It makes sense here because it is in the Prometheus ecosystem. However, the global flags are designed for a single API endpoint, not several systems.

TODOs:

1. Flags

  -H, --hostname string    Hostname of the Prometheus server (CHECK_PROMETHEUS_HOSTNAME) (default "localhost")

Maybe need another flag for setting the Alertmanager URI, since there could be an Alertmanager cluster that needs to be checked, OR we want to include data from the Alertmanager in the current alert subcommand then we would need to check two APIs.

But then we also need other flags for the call (auth and so on), pretty much all the global flags need to be duplicated.

2. Needs Tests

[martialblog]

@RincewindsHat Do you have any thoughts?