Added yesterday flag for pricing_sync

- Added yesterday flag for pricing_sync
Wouldn't it be better to pass 'yesterday' as today flag value? In your case it might be unintuitive ex. when someone pass `today='2014-11-11 -y' - then plugins will collect data for 2014-11-10. My suggestion is to "catch" yesterday value passed to today flag, ex:
```
if today == 'yesterday':
    today = datetime.date.today() - datetime.timedelta(days=1)
elif today:
    ...
```