Adding Timedelta parameter to time-based .rolling()
Starting with a simple DataFrame:

```python
n = 10
df = pd.DataFrame({'value': np.arange(n)},
                  index=pd.date_range('2015-12-24', periods=n, freq="D"))
```

I can invoke a time-based `.rolling()` with a string for the offset:

```python
df.rolling('3d').sum()
```

I would like to invoke it with a `Timedelta` since it is easier to arithmetically change the window length:

```python
df.rolling(pd.Timedelta(days=3)).sum()
```