Support generic StringToVar maps

[SOF3]

Support any Value implementation in map[string]V.

If the V implementation expects a comma, it is the responsibility of the user to recognize that it is impossible to provide a valid input.

[fredbi]

@SOF3 at the moment, this is not available. Hopefully, this repo will adopt generics some day.
Meanwhile, you may take a look at a generic implementation on top of pflag, that does just that:
https://github.com/fredbi/gflag