Feature request: option to set week starting on Sunday instead of Monday
#### Problem description

Currently, calling `pd.Series.dt.weekday` returns Monday = 0, Sunday = 6. It would be nice to have a settable option that allows Sunday to be the beginning of the week, e.g.

```python
my_series.dt.weekday  # Monday = 0, Sunday = 6
pd.options.week_start = 'Sunday'
my_series.dt.weekday  # Sunday = 0, Saturday = 6
```

Pandas version 1.0.1.