Use StringReplacer interface instead of strings.replacer

[thespags]

There exists StringReplacer already, but it's not available on a specific viper instance, only the singleton instance.

[CLAassistant]

All committers have signed the CLA.

[thespags]

Hi @sagikazarmark, is this something non-controversial to merge? There's a StringReplacer interface created for viper, that's used for the singleton instance, but on the viper call itself it requires the struct *strings.Replacer, which is harder to implement a matcher for.