refactor(toc): avoid using htmlTag

[SukkaW]

What does it do?

Fix #4181, an alternative to #4182.

By avoid using over complicated htmlTag() to gain better performance and fit the current test cases.

How to test

git clone -b refactor/og-remove-htmltag https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[SukkaW]

The benchmark performed by Travis CI shows the PR help improved generating speed by 4%.

[seaoak]

LGTM.