Added "relative" option to sails-linker task

This will allow the linker task to output script and style tags with relative paths, so that the output from `sails build` can be easily used in a subdirectory (useful when building re-usable components).  The "relative" option can be placed in the top-level "sails-linker" config in a Grunt file if you want all of the subtasks (devJS, devStyles, etc.) to use relative paths, or in any of the individual subtasks for more granular control.