ENH: add ignore_index to nlargest and nsmallest method
#### Describe the solution you'd like

Just like other methods where we select a subset or reorder, we have a `ignore_index` argument, to reset the index. 


```python
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({"A": range(10), "B": range(10, 20)})

In [3]: df.nlargest(n=2, columns="A")
Out[3]: 
   A   B
9  9  19
8  8  18

In [4]: df.nlargest(n=2, columns="A", ignore_index=True)
Out[3]: 
   A   B
0  9  19
1  8  18
```
