ENH: add ignore_index argument to DataFrame.explode / Series.explode
When we use `DataFrame.explode`  right now, it will repeat the index for each element in the iterator. To keep it consistent with the methods like `DataFrame.sort_values`, `DataFrame.append` and `pd.concat`, we can add an argument `ignore_index`, which will reset the index.


```python
df = pd.DataFrame({'id':range(0,30,10),
                   'values':[list('abc'), list('def'), list('ghi')]})
print(df)

   id     values
0   0  [a, b, c]
1  10  [d, e, f]
2  20  [g, h, i]

print(df.explode('values'))
   id values
0   0      a
0   0      b
0   0      c
1  10      d
1  10      e
1  10      f
2  20      g
2  20      h
2  20      i
```
Expected behaviour with addition of the argument:
```python
df.explode('values', ignore_index=True)

   id values
0   0      a
1   0      b
2   0      c
3  10      d
4  10      e
5  10      f
6  20      g
7  20      h
8  20      i
```
