Add `shift` to `dt` accessor
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

df = pd.DataFrame(
    {
        "date": pd.date_range("2020", freq="D", periods=10),
        "value": np.random.rand(10),
    }
)

shifted_dates = df.date.dt.shift(1)

```
#### Problem description

I always thought that the `dt` accessor should replicate more or less the functionality of a DatetimeIndex to a series or a column of a dataframe. If I can do it with a DatetimeIndex, I should be able to do it with a collection of datetimes too, right?

#### Expected Output

This is my workaround to obtain the expected output. It's not very nice.

```python
index = df.set_index("date").index
index.freq = index.inferred_freq

shifted_dates = df.date.copy()
shifted_dates[:] = index.shift(1)
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0.post20200127
Cython           : 0.29.14
pytest           : 5.3.5
hypothesis       : 5.4.1
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.3.2
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.48.0

</details>
