[TW-1536] Make eow/sow aware of the weekstart config option

_Michael Meier on 2015-02-12T19:43:48Z says:_

The date shortcuts eow/sow expand to dates correct for a weekstart of sunday, with sow producing next sunday 00:00 and eow producing next saturday 23:59. It would be nice if both abbreviations would take the "weekstart" rc option into account, so that they work correctly for users from countries where weeks go from monday to sunday, like Germany in my case.