code: function rawBundleFromFile (dir, filename) {
    return readFilePromise(path.join(dir, filename))
      .then(content => {
        return { dest: filename, raw: content };
      });
  }
docs: Given a directory and file in that directory, return a promise
  that resolves to a bundle that contains the filename and raw
  content.
  
  @param   {String}  dir       Valid path to directory containing Interlock bundles.
  @param   {String}  filename  Filename of bundle in dir.
  
  @returns {Promise}           Resolves to bundle matching input file.