Add base to use plugins in page format
Remove githubId

Enable hook for ebook
Enable css from plugins in page output
Use correct resources file for ebook
Fix correct folder for assets
Merge pull request #440 from GitbookIO/feature/ebookplugins

Apply plugins to ebook generation
Merge branch 'master' of github.com:GitbookIO/gitbook

Conflicts:
	lib/generate/site/index.js
从远程仓库中合并代码，更新app.js
更新说明文档