Move variables to book.json

This PR simply move 'variables' to the `book.json` (see https://github.com/GitbookIO/gitbook/commit/591d8fc9eef2c6727c3d66bbe50b833893731532)

This is more simple and let the user only use one configuration file to define its book. This method also made it simple to add in the future some transformations on configurations from book.json (and so variables).