API: allow option truncate for display.show_dimensions to only show dimensions if truncated (GH6457)

closes #6547

add the option 'trunctate'.
Should this be the new default?????

```
dfd = pd.DataFrame(np.arange(25).reshape(-1,5), index=[0,1,2,3,4], columns=[0,1,2,3,4])

# show dimensions only if truncated
with pd.option_context('display.max_rows', 2, 'display.max_columns', 2,  'display.show_dimensions', 'truncate'):
        print(dfd)
```

```
   0  1    
0  0  1 ...
1  5  6 ...
  .. ..    

[5 rows x 5 columns]
```

```
# show dimensions only if truncated
with pd.option_context('display.max_rows', 10, 'display.max_columns', 40, 'display.show_dimensions', 'truncate'):
        print(dfd)
```

```
    0   1   2   3   4
0   0   1   2   3   4
1   5   6   7   8   9
2  10  11  12  13  14
3  15  16  17  18  19
4  20  21  22  23  24
```