Support for multiple handlers with different routing keys

This change allows you to create multiple handlers with different routing keys, ie:

{
  "handlers": {
    "victorops": {
      "type": "pipe",
      "command": "/etc/sensu/handlers/victorops.rb --routingkey everyone",
      "severities": [
        "ok",
        "critical"
      ]
    },
    "victorops_database": {
      "type": "pipe",
      "command": "/etc/sensu/handlers/victorops.rb --routingkey victorops_database",
      "severities": [
        "ok",
        "critical"
      ]
    }
  }
}

Alternatively you can leave the --routingkey option off and use this handler exactly like before - this change is backwards compatible.
I think global variable here is too much for the purpose, instance variable would be enough.