API: allow multiple indices in Index.insert

Derived from #7299. `numpy.insert` can accept multiple elements / indices as of v1.8.
http://docs.scipy.org/doc/numpy/reference/generated/numpy.insert.html

Allow `Index` to accept multiple elements / indices. Currently, it raises `IndexError`.

```
import pandas as pd
idx1 = pd.Index([1, 2, 3, 4, 5])
idx1.insert([1, 2], 0)
# IndexError: invalid slice
```
