Ability to render documentation versions from git branch or tag

It would be awesome to have options to:
- render from only specified branches/tags
- specify url prefix for document version
- render some switcher (like for translations in _navbar.md)

----
something like this:
```js
window.$docsify = {
// ...
versions: {
'master': 'latest', // <-- branch 'master' will render by accessing url '/en/latest'
'1.5.1': '1.0', // <-- its just the tag '1.5.1' in git & no branch created (url '/en/1.0')
'2.0': '2.0' // <-- this is created branch (url '/en/2.0')
}
// ...
}
```

未指定文件、测试或入口点。首先定位现有的配置和文档渲染入口点，然后跟踪文档 URL 的解析方式。完成标准是支持所提议的 branch/tag 选择、URL 前缀和版本切换器，同时不破坏当前的渲染。

由索引模型根据 Issue 内容生成。