refactor: es6-fy

[SukkaW]

Check List

Please read and check followings before submitting a PR.

• I want to publish my theme on Hexo official website.
  • I have read the theme publishing doc.
  • name is unique.
  • link URL is correct.
  • preview URL is correct.
  • preview URL web site is rendered correctly.
  • Add a screenshot to source/themes/screenshots.
  • Screenshot filename is same as value of name.
  • Screenshot size is 800 * 500.
  • Screenshot file format is png.
• I want to publish my plugin on Hexo official website.
  • I have read the plugin publishing doc.
  • name is unique.
  • link URL is correct.
• Others (Update, fix, translation, etc...)