Add reference links support

This commit resolves #184
fix links error
Improve code syntax for links assignment
Fix #296: fix static links on windows
Improve font sizes and spacing on content
Improve meta tags for books
Improve navigation links design
Use base style for ebook instead of bootstrap
Show quiz inside ebook
Don't use cdn for ace
Fix title and description for pages in website
Fix GitbookIO/gitbook.io#69: copy cover files at multi-langs root
Fix style for main languages index page
Bump version to 0.5.1
Fix cover picture for multilangs book convertion to ebook
Fix font size for mono font on pdf output
Bump version to 0.5.2
Remove useless log
Allow external links in summary, fixes #300
Add link to gitbook.io in readme