publishGitbook() should check for existing gh-pages branch on github

[wabarr]

I think this might be preferable to current behavior of initializing an empty local repo and force pushing it onto github. The flow would be

• if gh-pages branch exists
• stash local changes
• pull from remote
• apply stashed changes
• push

This would preserve gh-pages commit history. It would also prevent forcible overwrite of other files in the gh-pages branch that are not used by gitbook, such as supplementary data files, or a CNAME file (which is necessary for custom domain name using github pages). Such files currently get forcibly overwritten in the current behavior, even if added to .gitignore file.

[wabarr]

I found your related discussion of this with ramnathv. Maybe another option is to have an option to add other files (like the CNAME file) when publishing to github. GitbookIO/gitbook#106 (comment)

[wabarr]

This pull request solves the problem I was having. #14