add multilingual selection plugin for gitbook

[billryan]

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
• rtCamp/gitbook-plugin-edit-link - make use of gitbook.toolbar.createButton API
• site/header.styl - hexojs/site - CSS for lang-select
• Plugins | GitBook Developers