feat(cache): option for cache rendered content

[SukkaW]

What does it do?

In #3756, @dailyrandomphoto disabled cache rendered content for generate and enabled for hexo s.

Enabling cache rendered content for hexo s will respond more quickly from second requests which is helpful when use hexo-server in production, but it also makes more difficult when developing theme or previewing other changes. So the config.server.cache is introduced in this PR. The documents for hexo-server will be updated after this PR is merged and released with hexo@4.3.

How to test

git clone -b server-development 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 increased (+0.0008%) to 97.14% when pulling a4719b4 on SukkaW:server-development into 2c50d3b on hexojs:master.

[coveralls]

Coverage increased (+0.0008%) to 97.14% when pulling a4719b4 on SukkaW:server-development into 2c50d3b on hexojs:master.

[curbengh]

Enabling cache rendered content for hexo s will respond more quickly from second requests which is helpful when use hexo-server in production, but it also makes more difficult when developing theme or previewing other changes.

This PR disable it by default though, shouldn't it be enabled for most users?

[SukkaW]

@curbengh

Most users and theme developers are using hexo-server as a live preview. Only few of them are using hexo-server to serve their website.

[curbengh]

Possibly supersedes hexojs/hexo-server#76