ENH: keyword to specify where to insert level with stack/unstack?
`unstack/stack` does insert the new level as the inner-most level (last level). Would it be useful to have a keyword to specify where to insert the new level. E.g to have it as the first level you now have to do a `reorder_levels` and `sort_index`:

```
In [47]: df = pd.DataFrame({'A': np.zeros(5), 'B': np.zeros(5)}, index=pd.date_range('2012-01-01', periods=5, freq='D'))
    ...: df = pd.concat([df]*3, keys=['x1', 'x2', 'x3'])

In [48]: df
Out[48]: 
               A  B
x1 2012-01-01  0  0
   2012-01-02  0  0
   2012-01-03  0  0
   2012-01-04  0  0
   2012-01-05  0  0
x2 2012-01-01  0  0
   2012-01-02  0  0
   2012-01-03  0  0
   2012-01-04  0  0
   2012-01-05  0  0
x3 2012-01-01  0  0
   2012-01-02  0  0
   2012-01-03  0  0
   2012-01-04  0  0
   2012-01-05  0  0

In [49]: df.unstack(0)
Out[49]: 
             A           B        
            x1  x2  x3  x1  x2  x3
2012-01-01   0   0   0   0   0   0
2012-01-02   0   0   0   0   0   0
2012-01-03   0   0   0   0   0   0
2012-01-04   0   0   0   0   0   0
2012-01-05   0   0   0   0   0   0

In [50]: df.unstack(0).reorder_levels([1,0], axis=1).sort_index(axis=1)
Out[50]: 
            x1     x2     x3   
             A  B   A  B   A  B
2012-01-01   0  0   0  0   0  0
2012-01-02   0  0   0  0   0  0
2012-01-03   0  0   0  0   0  0
2012-01-04   0  0   0  0   0  0
2012-01-05   0  0   0  0   0  0
```
