Implement JSON API to allow easier interfacing

### Environment

* ZeroNet version: from the future
* Operating system: All

### Describe the proposal:

Have an JSON API server to allow easier programmatic interaction with ZeroNet.
i.e. http://127.0.0.1:43110/api/sites?access_token=fcc0a328b7060b9394a0a73267c96bbb

#### Steps to reproduce:

1. Define the command set you would like to implement.
2. Write some **client side** code on a piece of paper before you start implementing anything. Since hand writing takes time, if it takes a lot of writing to use the interface, you should reconsider the API design.
3. Implement the interface, so that the example client code works.
4. Add a command line option for turning it on and off.

#### Benefits:

Developers need not simulate WebSocket requests anymore.

首先定义命令集，并为示例 JSON API 端点编写拟议的客户端调用。然后确定应如何实现接口，以及如何通过命令行选项公开该接口；完成的标准是示例客户端交互能够正常工作，并且可以启用或禁用 API。

由索引模型根据 Issue 内容生成。