code: function frontmatter(opts){
    var options = {}, grayMatterOptions = {};
    Object.keys(opts || {}).forEach(function(key){
      if (key === 'namespace'){
        options[key] = opts[key];
      } else {
        grayMatterOptions[key] = opts[key];
      }
    });
  
    return each(function(file, filePath){
      extractFrontmatter(file, filePath, options, grayMatterOptions);
    });
  }
docs: Metalsmith plugin to extract metadata from frontmatter in file contents.
  
  @param {Object} options - `Namespace` is used, the rest is passed on to `gray-matter`
  @return {Function}