code: function(scrollTop) {
          var lineCount = this.$textarea.val().split('\n').length,
              numbers = '1';
          for (var i = 2; i <= lineCount; i++) {
              numbers += '\n' + i;
          }
          this.$lineNumber.html(numbers);
          this.scrollLineNumber(scrollTop);
      }
docs: 绘制行号
  @param {Number} scrollTop 滚动高度
  @private