Allow CUSTOM event-types

[nathanlcarlson]

To test the Nagios integration I attempted to send a custom notification on a service via the Nagios GUI. Currently "CUSTOM" is not an allowed event-type.

pd-nagios: error: argument -t/--event-type: invalid choice: 'CUSTOM' (choose from 'PROBLEM', 'ACKNOWLEDGEMENT', 'RECOVERY')

[NargiT]

To what CUSTOM should be mapped then ? Currently this is the mapping between nagios -> pagerduty

"PROBLEM": "trigger",
"ACKNOWLEDGEMENT": "acknowledge",
"RECOVERY": "resolve"