code: function processFile(frontMatter) {
    return (file, callback) => {
      let meta = {};
      let content = file.data;
      const match = content.match(FRONT_MATTER_DELIMITER_RE[frontMatter]);
      if (match !== null) {
        try {
          meta = parseFronMatter(content.slice(0, match.index), frontMatter);
        } catch (error) {
          console.error(`${file.path}: Invalid front matter`);
          async.nextTick(callback, error);
          return;
        }
        content = content.slice(match.index + match[0].length);
      }
  
      meta.content = content;
      async.nextTick(callback, null, Object.assign({}, file, { meta }));
    };
  }
docs: Process a single file.