ENH: follow up on indx-repr
follow up to #9901
- also splitting the attributes part over multiple lines if this is very long
- for truncated output: now hard-code at 10 elements, but this could maybe also be 1 line? (and possibly with a minimum number of elements for longer strings) Because now, small integers don't fill the full line with 10 elements, and eg floats there is one element on the second line:

This is a function of the `display.width`. But the truncated output should restrict to a single-line if this happens.

```
In [6]: pd.set_option('display.width',100)

In [7]: Index(np.arange(400.))
Out[7]: 
Float64Index([  0.0,   1.0,   2.0,   3.0,   4.0,   5.0,   6.0,   7.0,   8.0,   9.0, 
              ...
              390.0, 391.0, 392.0, 393.0, 394.0, 395.0, 396.0, 397.0, 398.0, 399.0],
             dtype='float64', length=400)

In [8]: pd.set_option('display.width',80)

In [9]: Index(np.arange(400.))
Out[9]: 
Float64Index([  0.0,   1.0,   2.0,   3.0,   4.0,   5.0,   6.0,   7.0,   8.0,
                9.0, 
              ...
              390.0, 391.0, 392.0, 393.0, 394.0, 395.0, 396.0, 397.0, 398.0,
              399.0],
             dtype='float64', length=400)
```

This could eg easily be restricted to two times one line.
