Convert string slice to string using strings.Join(s, ",").

[eiffel-fl]

Hi!

In this PR, I added support to convert string slice to string using strings.Join(s, ",").
I was experimenting yesterday with some code based on spf13/viper and was surprised to get an empty string as result of calling GetString() on a config key I know was a string slice.
Moreover, I did not get any error message, as GetString() did not return one:
https://github.com/spf13/viper/blob/551e8939be1e18c45accb6d144e6c19e46ee24e1/viper.go#L785-L787

As I think this code may be useful for other people, I decided to open a PR.
Please let me know what you think about it.

Best regards.

[CLAassistant]

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