Add additional optOut key

[aeisenberg]

This is a feature request to add an additional, optional optOut key. My understanding is that the recommended way to use this extension is to ensure two configuration options are enabled before sending any telemetry. The first one is the global telemetry.telemetryEnabled option and the second one is specific to your extension. This enables someone to opt out of your telemetry while still opting in to vscode's global telemetry.

My proposal is to add an extra optional string parameter to the TelemetryReporter constructor. This parameter points to a config option that must also be enabled in order for telemetry to be sent.

The change is fairly straight forward and I'm happy to submit a PR if you think this is a reasonable direction to go.

[lramos15]

This does sound interesting but I'm not sure if optOut is the best word since we have telemetryEnabled where true is enabled and false isn't. Where optOut makes it sound like true is disabled and false isn't. How do extensions go about deciding which way their switch goes?