BUG: step argument in the slice StringMethod not working
The `step` argument in the `slice` StringMethods has no effect:

```
In [18]: s = pd.Series(['abcde', 'fghij'])

In [19]: s.str.slice?
Definition:  s.str.slice(self, start=None, stop=None, step=1)
Slice substrings from each element in array

Parameters
----------
start : int or None
stop : int or None

Returns
-------
sliced : array

In [22]: s.str.slice(step=1)
Out[22]: 
0    abcde
1    fghij
dtype: object

In [23]: s.str.slice(step=2)
Out[23]: 
0    abcde
1    fghij
dtype: object

In [24]: s[0][::2]
Out[24]: 'ace'
```
