ENH: Method to recover IntervalIndex when reloaing plain-text files
When an IntervalIndex is written to a plain-text format (csv or with `to_string()`) the output is well-formatted.  If I now reload that CSV file, the IntervalIndex is now recreated as a series / index of strings.  

A useful feature would be to convert that string index back into an IntervalIndex.  This could potentially be done automatically in the `read_csv` and `read_table` methods.

I've implemented such a function into my own code and extracted it to this gist:
https://gist.github.com/benkrikler/cfa0f3703358d5bd7b5abd5fae36db29

I'd need some guidance, but I'd be happy to try turning this into a PR if it were wanted.