Feature request: first & last for rolling()
#### Code Sample, a copy-pastable example if possible

```python
# This works:
df1 = df.resample(to_freq,
                  closed='left',
                  kind='period',
                   ).agg(OrderedDict([('Open', 'first'),
                                      ('Close', 'last'),
                                                        ]))

# This doesn't:
df2 = df.rolling(my_indexer).agg(
                 OrderedDict([('Open', 'first'),
                              ('Close', 'last') ]))
>>> AttributeError: 'first' is not a valid function for 'Rolling' object

df3 = df.rolling(my_indexer).agg(
                 OrderedDict([
                              ('Close', 'last') ]))
>>> AttributeError: 'last' is not a valid function for 'Rolling' object

```
Please, could 'first' and 'last' be made available for rolling window?
Thanks for your support!
Bests,
