ENH: add display.max_seq_items to limit len of pprinted long sequences
There's a circuit-breaker in higher layers of the display code,
but just in case.

``` python
n [2]: pd.options.display.max_seq_items=10

In [3]: pd.core.common.pprint_thing(range(10000))
Out[3]: u'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ...]'
```
