Add extend_files to config

[Paweł Knuth]

I have an issue where i need to add CNAME file to git repository in main directory, however hexo deploy always replace this file, i wish there could be way to add files that will be included in commit along with directories.

[Sukka]

Just add a CNAME file in source directory will do.

[Paweł Knuth]

Thanks @SukkaW, works