ENH: Pandas ewm with fixed window / offset
**Problem & Proposed feature**
Pandas ewm function works similar to the pandas expanding function in that it rolls over the whole dataframe. In my case however, I need to specify a fixed window or offset over which the ewm function is applied. In other words, an ewm function with a cutoff that would work similar to the 'window' parameter in the pandas rolling function.

**Alternatives considered**
My workaround to the problem is as follows (however extremely inefficient):

```
df = pd.DataFrame({'a': np.random.randint(5, size=24),
                   'b': ["S", "A"] * 12,
                   'c': pd.date_range(start='1/1/2018', end='12/12/2018', freq='15D')})

df.groupby('b').rolling('60d', on='c')['a'].apply(lambda x: x.ewm(halflife='15d', times=x.index).mean().tail(1))
```
In the example above, I am trying to do an exponentially-weighted moving average, where decay is specified in terms of halflife on a datetime column. The rolling apply ensures that the maximum offset is '60d' which is the feature I propose to be added directly in the ewm function.

So in summary, I propose a window parameter to be introduced to the ewm function which limits the ewm calculation to a fixed window or offset.