in-place dataset update
### Motivation

For the circumstance that I creat an empty `Dataset` and keep appending new rows into it, I found that it leads to creating a new dataset at each call. It looks quite memory-consuming. I just wonder if there is any more efficient way to do this.

```python
from datasets import Dataset

ds = Dataset.from_list([])

ds.add_item({'a': [1, 2, 3], 'b': 4})
print(ds)
>>> Dataset({
>>>     features: [],
>>>     num_rows: 0
>>> })

ds = ds.add_item({'a': [1, 2, 3], 'b': 4})
print(ds)
>>> Dataset({
>>>     features: ['a', 'b'],
>>>     num_rows: 1
>>> })
```

### Feature request

Call for in-place dataset update functions, that update the existing `Dataset` in place without creating a new copy. The interface is supposed to keep the same style as PyTorch, such as the in-place version of a `function` is named `function_`. For example, the in-pace version of `add_item`, i.e., `add_item_`, immediately updates the `Dataset`.

```python
from datasets import Dataset

ds = Dataset.from_list([])

ds.add_item({'a': [1, 2, 3], 'b': 4})
print(ds)
>>> Dataset({
>>>     features: [],
>>>     num_rows: 0
>>> })

ds.add_item_({'a': [1, 2, 3], 'b': 4})
print(ds)
>>> Dataset({
>>>     features: ['a', 'b'],
>>>     num_rows: 1
>>> })
```

### Related Functions

* `.map`
* `.filter`
* `.add_item`