code: function chooseDataSource(dataSource) {
          if (dataSource === 'json') {
              return function(file) {
                  return JSON.parse(file.contents.toString());
              };
          } else if (dataSource === 'vinyl') {
              return function(file) {
                  file[bodyAttribute] = file.contents.toString();
                  return file;
              };
          } else if (dataSource === 'data') {
              return function(file) {
                  var data = file.data;
                  data[bodyAttribute] = file.contents.toString();
                  return data;
              };
          } else if (typeof dataSource === 'function') {
              return dataSource;
          } else {
              throw pluginError('Unknown dataSource');
          }
      }
docs: Return a function to turn a file object into a JSON-like data object