Set a default offset value

[samaybar]

It would be helpful to be able to set a default offset value for time series run with the exporter. Most of the time if you are doing an offset you would want it to apply to all time series being run, so it would be more efficient to specify it globally.

[haganbt]

@samaybar - great idea. I will not be adding any new features to this repo as the new version is WIP. Added here - haganbt/pepp#16