API: add ignore_index keyword to .sort_* & .drop_duplicates
I see lots of code that goes like this

```
In [17]: df = pd.DataFrame({'A': [1, 2, 3]})                                                                                                                                        

In [18]: df                                                                                                                                                                         
Out[18]: 
   A
0  1
1  2
2  3

In [19]: df.sort_values('A', ascending=False)                                                                                                                                       
Out[19]: 
   A
2  3
1  2
0  1

In [20]: df.sort_values('A', ascending=False).reset_index(drop=True)                                                                                                                
Out[20]: 
   A
0  3
1  2
2  1
```

might be nice from an API / consistency perspective to add a ``ignore_index=False|True`` keyword to ``.sort_values()``, and ``.drop_duplicates()`` that does the reset in-line; this would give consistency similar to a ``pd.concat([......], ignore_index=True)`` operation which users are very familiar
