Enhancement: The shift function should have an optional argument allowing to extend the dataframe i.e. for forecasts
I believe using pandas dataframes to work with forecasts is a pretty standard use case. Therefore I am missing a convenient method to extend the dataframe in its length while shifting, instead of loosing values. Currently the only way to do this is something like:

```python

    df_ext = pd.DataFrame(index=pd.date_range(df.index[-1], periods=8, closed='right'))
    df2 = pd.concat([df, df_ext], axis=0, sort=True)
    df2["forecast"] = df2["some column"].shift(7)

```

My proposal would be to add an extra argument to the shift function i.e. something along the lines:

```python
df["x"].shift(7, inplace=True, extend_index_freq='D')
```