Provide a default GatewayDiscordClient bean

[stelitop]

Currently, creating a configuration with a custom GatewayDiscordClient bean is necessary to use mad4j. A default implementation should be provided that just starts the bot online.

The bot token and starting status should be configurable. Since the token should be stored securely, it should not be stored in the application.properties file. The rest of the configurations can be.

The bot token should be stored in a separate file that contains the token. The user should add that file to their .gitignore.