ENH: Add Index.fillna
Related to #6194. There seems no direct way to replace `NaN`s in `Index`?

I think a point is what types are acceptable via `value`.
- scalar: OK
- dict: NG (or accept {iloc : value} dict?)
- Series : NG
- DataFrame : NG
- http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.fillna.html
