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

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`.