A new agent must comply with these simple implementation rules:
1) Implement the `AgentName()` method. It just returns the name of the agent for logging purposes.
2) Implement one or more of the `*Retriever()` interfaces. That's where the agent's logic resides.
3) Register itself (in its `init()` function).
For an agent to be used it needs to be listed in the `Agents` config option (default is `"lastfm,spotify"`). The order dictates the priority of the agents