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
```

@jseabold @jorisvandenbossche @TomAugspurger @cpcloud 
 Looks good on a quick glance.
 small detail to nitpick, but `truncated` instead of `truncate` sounds better to me (as the option), as it is not really involved in the action of truncating, only the state of being truncated
 @jorisvandenbossche I agree it sounds better, except, that `large_repr` uses `truncate` 

and wanted to be consistent, but ok if you feel strongly
 changed to use 'truncated' & example in v0.14.0.txt simpler
 Ah, but if `large_repr` already uses the other, I am not feeling strongly about it.

@jreback about if this closes the issue that @jseabold brought up in #6547, he can maybe better say it himself, but I don't think it really does solve his issue he encountered in statsmodels (the use of dataframes to give back the overview result of a model, for which these dimensions are a bit superfluous/distracting). But as also said on the mailing list, not everybody's needs can be satisfied at the same time. 
And this PR provides in any case all needed architecture. Given this, the discussion in the issue/on the mailing list, is only about what should be the default value of `display.show_dimensions`: `True` or `truncated`? `True` would be compatible with 0.13, `truncated` would be compatible with <=0.12 (for small dataframes).
 ok..put back to 'truncate'.

like @jseabold to comment (and maybe should send to ML). but I think let's make the default 'truncate'.?
 Posted on the [ML](https://groups.google.com/forum/#!topic/pydata/wyhbADpsUss). I tentatively vote for the default to be `truncate(d)`. That really does fix the statsmodels issue.
 thanks @TomAugspurger 
