feat(permalink): pretty_urls.trailing_html

[curbengh]

What does it do?

Continuation of #3691

When pretty_urls.trailing_html is disabled, permalink like example.com/foo/bar.html is transformed into example.com/foo/bar/. Trailing slash is appended for now, pretty_urls.trailing_slash can be introduced in later PR.

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 trailing-html 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 increased (+0.01%) to 97.132% when pulling 335d9cc on curbengh:trailing-html into 6216d05 on hexojs:master.

[SukkaW]

Since codeclimate detects too many duplicated code blocks, we could bring up a pretty_url(input, [options]) in hexo-util.

[curbengh]

hexojs/hexo-util#152