Feature Request: add a startday argument to pandas.DatetimeIndex.dayofweek?
Not sure if this is feasible or if there is support for it.

https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DatetimeIndex.dayofweek.html
gives the day of the week with Monday=0, Sunday=6.

For my application I would like Sunday=0 and Saturday=6.

I wonder if there could be a startday argument to `dayofweek`? e.g. `dayoftheweek(startday='Sunday')` with `dayoftheweek(startday='Monday')` being the default.

I used a workaround in my case:
```
df['weekday'] = df.index.dayofweek 
df_weekday_mean = df.groupby('weekday').mean()
df_weekday_mean = df_weekday_mean.iloc[np.arange(-1, len(df_weekday_mean)-1)].reset_index()
```

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: 3.9.1
pip: 10.0.1
setuptools: 40.4.3
Cython: 0.29
numpy: 1.15.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.0.1
sphinx: 1.8.1
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.9
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.12
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
