Add a config file for secrets, api key, etc.

[DblK]

I would like to be able to setup configuration for various objectives such as:

• Having Youtube apikey inside config
• Having the type of the endpoing (v1 vs stage)
• Have the domain from which we will serve the app
• Have the list of user and their apikey to secure a bit the api
• ...

To manage the configuration, we will use github.com/spf13/viper.