API: Series.str.match == extract?
This is the whatsnew example. I think for compat, the 3rd entry should be `(np.nan, 3)` ?

```
In [1]: Series(['a1', 'b2', '3']).str.match('(?P<letter>[ab])?(?P<digit>\d)')
Out[1]: 
0       (a, 1)
1       (b, 2)
2    (None, 3)
dtype: object
```
