docs: function xinclude(file, encoding, callback) {
      if (!file) {
          throw new PluginError('gulp-xinclude', '文件为没有对应的文件');
      }
  
      if(file.isStream()) {
          throw new PluginError('gulp-xinclude', '只支持html文件中<!--xinclude command=xxx-->类型指令。');
      }
  
      processFile(file, encoding)
      this.push(file);
      callback();
  }