Possibility to choose European OpsGenie API Endpoint - OPSGENIE_URL

[Maros Mitucha]

Hello,

due to EU regulation there is dedicated European OpsGenie API endpoint: https://api.opsgenie.com/v2/alerts/
see https://docs.opsgenie.com/docs/alert-api
& https://docs.opsgenie.com/docs/european-service-region

Variable is OPSGENIE_URL
https://github.com/sensu-plugins/sensu-plugins-opsgenie/blob/master/bin/handler-opsgenie.rb#L16

It would be very useful if there is a possibility for overwriting API endpoint. Otherwise the API response is always "unauthorized".

By the way the plugin is great,
thank you very much for work.