Expand options for exporting schedule

Currently the only option for exporting a schedule (https://github.com/opsgenie/opsgenie-go-sdk-v2/blob/master/schedule/schedule.go#L75) is by giving a schedule and a file path as a parameter, which the function combines to export to that filepath + scheduleidentifier.

This is quite restrictive. Suggested additions are: Export to []byte and export to specific file path.