Add key option to sort methods

[SethMMorton]

One feature that I am missing from dataframes is to sort with a custom key function, similar to the builtin sorted or list.sort. I see there is some effort to unify sorting in issue #8239, so it might be a nice to add this powerful functionality to pandas while the sorting API is being overhauled.

[jreback]

pls show an example of what/how this would be used as generally you can simply pass in the column name.

[jreback]

dupe of #3942
(you can add an example to that issue)

[SethMMorton]

I have added the specific example to the dupe. Thanks!