Can't use named aggregation with resample. 
#### Code Sample, a copy-pastable example if possible
```python
import pandas as pd
import numpy as np
from functools import partial

df = pd.DataFrame(
    np.random.randn(1000, 3), 
    index=pd.date_range('1/1/2012', freq='S', periods=1000), 
    columns=['A', 'B', 'C']
)
dfg = df.resample('3T').agg(
    {'A': [
        partial(np.quantile, q=.9999), 
        partial(np.quantile, q=.90),
    ]}
)
```
#### Problem description
Resample, unlike groupby, has no ability to do named aggregation. This means if you use a function twice on the same column you get an error saying:

``pandas.core.base.SpecificationError: Function names must be unique, found multiple named quantile``. 

It would be very useful to make the resample agg interface similar to the groupby agg interface. An example of the named aggregation is here: https://pandas.pydata.org/pandas-docs/stable/user_guide/groupby.html#named-aggregation

#### Expected Output

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8
pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.6.2
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
