code: function plugin(options){
    options = options || {};
    var keys = options.keys || [];
  
    return function(files, metalsmith, done){
      setImmediate(done);
      var md = new MarkdownIt(options);
      Object.keys(files).forEach(function(file){
        if (!markdown(file)) return;
        var data = files[file];
        var dir = dirname(file);
        var html = basename(file, extname(file)) + '.html';
        if ('.' != dir) html = dir + '/' + html;
  
        var str = md.render(data.contents.toString());
        data.contents = new Buffer(str);
        keys.forEach(function(key) {
          data[key] = md.render(data[key]);
        });
  
        delete files[file];
        files[html] = data;
      });
    };
  }
docs: Metalsmith plugin to convert markdown files.
  
  @param {Object} options (optional)
  @property {Array} keys
  @return {Function}