How to: Putting Values in Viper
Viper can read from multiple configuration sources and merges them together
into one set of configuration keys and values.

Viper uses the following precedence for merging:

 * explicit call to `Set`
 * flags
 * environment variables
 * config files
 * external key/value stores
 * defaults

> **NOTE** Viper configuration keys are case insensitive.