ENH: support of pandas.DataFrame.hist for datetime data
- [x] closes #32590
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# Update

```
import numpy as np
from pandas import DataFrame, to_datetime
from datetime import timedelta

df = DataFrame(
    {
        "a": np.random.rand(10),
        "b": [timedelta(np.random.randn()) for _ in range(10)],
        "c": to_datetime(
            np.random.randint(
                1582800000000000000, 1583500000000000000, 10, dtype=np.int64
            )
        ),
        "d": to_datetime(
            np.random.randint(
                1582800000000000000, 1583500000000000000, 10, dtype=np.int64
            ),
            utc=True
        ),
    }
)

df.dtypes
a                float64
b        timedelta64[ns]
c         datetime64[ns]
d    datetime64[ns, UTC]
dtype: object

df.hist(xrot=90, figsize=(9, 6))
array([[<AxesSubplot:title={'center':'a'}>,
        <AxesSubplot:title={'center':'c'}>],
       [<AxesSubplot:title={'center':'d'}>, <AxesSubplot:>]], dtype=object)
```
![download](https://user-images.githubusercontent.com/21543236/93713409-3f0d8580-fb8e-11ea-934a-a6e302a7b14d.png)

# ~~WIP~~

Since there's no response for several days in https://github.com/pandas-dev/pandas/issues/32590, I decide to open this PR for further discussion.
There's two feasible options:

Plan A:
As is shown in the files changed, `data._get_numeric_data()` is modified to `data._get_numeric_or_datetime_data()`, and I will work on related tests as the next step.

Plan B, this may be a fotfix according to https://github.com/pandas-dev/pandas/issues/32590#issuecomment-687477764:
```
column_dt = data.select_dtypes(include='datetime64[ns]')
if len(column_dt) > 0:
    data[column_dt] = data[column_dt].astype(np.int64)
data = data._get_numeric_data()
if len(column_dt) > 0:
    data[column_dt] = data[column_dt].astype('datetime64[ns]')
```
Also, related tests will be modified.

Any comment / thought is welcomed.