How to: Watching and Re-reading Config Files
Gone are the days of needing to restart a server to have a config take
effect--Viper powered applications can read an update to a config file while
running and not miss a beat.

It's also possible to provide a function for Viper to run each time a change
occurs.

```go
// All config paths must be defined prior to calling `WatchConfig()`
viper.AddConfigPath("$HOME/.appname")

viper.OnConfigChange(func(e fsnotify.Event) {
	fmt.Println("Config file changed:", e.Name)
})

viper.WatchConfig()
```