feat(permalink): pretty_urls.trailing_html

[curbengh]

What does it do?

Continuation of #3691.
Utilize prettyUrls() introduced in hexojs/hexo-util#152 (credit @SukkaW)
Supersede #3893

When pretty_urls.trailing_html is disabled, permalink like example.com/foo/bar.html is transformed into example.com/foo/bar.

The option doesn't apply to url with trailing index.html, e.g. example.com/foo/bar/index.html. I don't think example.com/foo/bar/index/ is valid.

How to test

git clone -b pretty_urls-util https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage decreased (-0.009%) to 97.13% when pulling e6955d3 on curbengh:pretty_urls-util into 219fded on hexojs:master.

[curbengh]

There's a failing test due to incomplete #3921, I'll fix it.

Edit: #3924