Timestamp.strftime(): missing support for nanoseconds
When parsing text into a `Timestamp` object we can specify a format string. Currently `%f` is documented with

> note that "%f" will parse all the way up to nanoseconds

See https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.to_datetime.html, in particular the description of the `format` parameter. The note about `%f` was added in this patch: https://github.com/pandas-dev/pandas/pull/8904


The fact that we can parse text using nanosecond precision is great, and here I make use of that behavior (showing two methods yielding the same result):

```
# Implicit format:
>>> t = pd.to_datetime('2019-10-03T09:30:12.133333337')
>>> t
Timestamp('2019-10-03 09:30:12.133333337')


# Explicit format using %f:
>>> t = pd.to_datetime('2019-10-03T09:30:12.133333337', format='%Y-%m-%dT%H:%M:%S.%f')
>>> t
Timestamp('2019-10-03 09:30:12.133333337')
````

But when I now want to invert that process using `strftime()`  then the fractional part is truncated to microsecond precision:

```
>>> t.strftime('%Y-%m-%dT%H:%M:%S.%f')
'2019-10-03T09:30:12.133333'
```

On the one hand this is inconsistent with the meaning of `%f` while parsing. On the other hand it corresponds to what's documented in Python's stdlib documentation (which says that `%f` means "Microsecond as a decimal number, zero-padded on the left.").

In any case, I think it would make sense to have a format string specifier that allows us to turn the timestamp into a string with nanosecond precision.

If I am not mistaken, we otherwise have to work around the absence of that format specifier by using the `nanosecond` property:

```
>>> t.nanosecond
337

>>> t.strftime('%Y-%m-%dT%H:%M:%S.%f') + str(t.nanosecond)
'2019-10-03T09:30:12.133333337'
```

Do you agree that we should have a format specifier for that? Or do we have one, and it's just not documented?



<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.7-200.fc30.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.2
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : 4.41.3
sphinx           : 2.2.0
blosc            : 1.8.1
feather          : None
xlsxwriter       : 1.2.2
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.2.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : 0.3.5
scipy            : 1.3.1
sqlalchemy       : 1.3.10
tables           : 3.6.0
xarray           : 0.14.0
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.2

</details>
