Add OpsGenie Connector and Converters

[willmarks]

We would like to build a connecter and converters for OpsGenie. To start, this should be a similar connector to that of PagerDuty as we are interested in incident data.

Data to retrieve:

• Incidents
• Users

API Docs

Note: Be aware of rate-limiting when using node.js API client as it does not appear to handle 429 responses

[huongtn]

@willmarks because node.js API client does not support incident query, so I just use axios.