useminPrepare accepts array of root directories

Added the option set an array of `root` directories for `useminPrepare`.

Use case:
I often use `grunt-contrib-connect` as my local static file server while development. `grunt-contrib-connect` let me set an array of `base` path which will be used as the root for all files. In this way I can logically separate my own source files and vendor files (e.g. `bower_components`) in my file system, but my web server thinks they are in the same root.