Feature Request: `apply` method to `Index` objects
I recently started using `apply` on my `DataFrames` and `Series`.  It would be awesome to have this functionality on `Index` objects.  I usually have to rename my indices a lot. 

```
D_obsv_meta = dict(of-label-mapping)
DF_raw.index.apply(lambda x: D_obsv_meta[x])
```
