function finish(hash, callback) {
    var result = JSON.stringify(hash, null, 4);
    fs.writeFile(RESULT_PATH, result, function (err) {
        if (err) return callback(err);
        callback(null, hash);
    });
}
Write sentiment score hash to disk.
@param  {object}   hash     Result hash
@param  {Function} callback Callback
@return {void}