add multilingual selection plugin for gitbook

Since https://github.com/GitbookIO/plugin-versions rely on GitBook API and is more suitable for multi-version software API, we need a multilingual selection plugin for gitbook pages.

Some JS or CSS we can refer:
- [GitbookIO/plugin-versions](https://github.com/GitbookIO/plugin-versions)
- [rtCamp/gitbook-plugin-edit-link](https://github.com/rtCamp/gitbook-plugin-edit-link) - make use of `gitbook.toolbar.createButton` API
- [site/header.styl - hexojs/site](https://github.com/hexojs/site/blob/master/themes/navy/source/css/_partial/header.styl) - CSS for lang-select
- [Plugins | GitBook Developers](https://developer.gitbook.com/plugins/index.html)

Start with the GitBook plugin documentation and the plugin-versions and edit-link examples, focusing on the gitbook.toolbar.createButton API. Review site/header.styl for the language selector styling; done means a GitBook plugin provides multilingual page selection with the referenced UI behavior.

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