SetFs useless when not using default Viper instance

[progrium]

When creating an instance of Viper (such as writing tests), you can use SetFs to work with, say, an in-memory filesystem. However, Viper will not use it when looking up a configuration file. searchInPath uses a util.go function called exists that is hardcoded to the global v instance.

This can be fixed by simply letting exists take an instance of a filesystem to use.