Relative build path support

[mirari]

Fixed the problem that the URLs of fonts and images in the CSS files are incorrect when assetsPublicPath set to ./ in production environment.
Now the 'dist' directory can be visited by any URL root path at the same time.
For example:
visit http://192.168.1.101/project/ in intranet,
and visit http://project.example.com/ in internet.

修正assetsPublicPath被设置成./时，CSS文件中的图片、字体等引用地址错误的问题。
现在项目可以支持发布到任意层级的目录。比如在维护主机的内网环境下使用 http://192.168.1.101/project/访问主页，同时又可以在广域网下以http://project.example.com/这样的根路径访问同一主机。
如果项目地址发生变动，比如从http://www.example.com/project-a/变成了http://www.example.com/project-b/sub-a/，也仅仅只需要调整项目文件目录，而不需要修改assetsPublicPath并重新编译打包。