Make `DataFrame.pivot` accepts a list of column names as values
#### Problem description

Currently `DataFrame.pivot` accepts either `None` or a column name for the option `values` but not a list of column names.
```python
# this works
df.pivot(values = None)
df.pivot(values = 'column_name')
```
```python
# this doesn't work
df.pivot(values = ['cn1', 'cn2'])
```
 It is not convenient if I want to create a pivot table for multiple columns at the same time. Of course, we can pass `None` to `values` and then select columns we want to. But this is not as convenient. 

