feat(toc_helper): add min_depth option

[SukkaW]

What does it do?

Add min_depth option.

The unit test for toc() has been refactored as well.

Supersedes and closes #3929

How to test

git clone -b toc_min_depth 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.

[coveralls]

Coverage remained the same at 97.103% when pulling 0cfb532 on SukkaW:toc-min-depth into a381dda on hexojs:master.

[curbengh]

tested

[noraj]

Thanks a lot