refactor(hexo/index.js): Class syntax

[curbengh]

What does it do?

Continuation of #3754. Inspired by hexojs/warehouse#52.

Using lebab as suggested by @deepnum mentioned in that PR, this PR simply lebab --transform class index.js -o class.js and then hoist the arrow functions to the top; I might miss something.

Other files in /lib/hexo can be refactored to this style as well, but since each file has >100 lines, I prefer to do each of them in a separate PR for easier review.

How to test

git clone -b refactor-class https://github.com/refactor-class/hexo.git
cd hexo
npm install
npm test

Tested on a test blog (both hexo g & s).

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.01%) to 97.255% when pulling 4022108 on curbengh:refactor-class into 52adf39 on hexojs:master.