Add grunt as alterntive to hard links

Hello,

As a less invasive replacement to my original proposal ( https://github.com/honestbleeps/Reddit-Enhancement-Suite/pull/711) this pull simply adds in a grunt.js & package.json which will allow any developers who want to, to use grunt rather than creating hardlinks when developing on RES.

When running the script will automatically detect any changes to files in lib and automatically copy them to the relevant extension folder (for use in testing).

Thanks,
Carl