[Discussion] New plugin system

Moving the discussion from the PR #1430 (read from [this comment](https://github.com/HelloZeroNet/ZeroNet/pull/1430#issuecomment-420003512) onwards) to here.

*Topic*: How to write a plugin architecture for ZeroNet in such a way that everyone can install plugins from "plugin store" in sites?

*Possible choices* are to have one-click install with current plugin system, change plugin system to a more modular one, and to sandbox plugin execution in some way.

Please read the other PR before writing here.

首先阅读 PR #1430，尤其是 issue 中要求查看的链接评论及后续讨论。将提议的一键安装、模块化插件系统和沙箱选项与 ZeroNet 当前的插件系统进行比较。只有在就具体的架构和范围达成一致，而不是仅仅进行讨论后，这项工作才算完成。

由索引模型根据 Issue 内容生成。