API/ENH: Add setter methods to vectorized string API
This would be nice. The idea is simple: be able to modify the elements of vectorized string list or tuples, e.g., after splitting them, a la 

``` python
s = Series(['a/b/c', 'd/e/f'])
sp = s.split('/')
sp.str[0] = 'x'
assert np.all(sp.str[0] == 'x')

# as well as 
sp.str.set(0, 'y')
assert np.all(sp.str[0] == 'y')
```
