perf(external_link): cache config

[SukkaW]

What does it do?

This PR add cache for 2 values:

• EXTERNAL_LINK_(POST|SITE)_ENABLED

Since hexo will only run filter for post or site, it is important to return earlier for another one.

• EXTERNAL_LINK_(POST|SITE)_CONFIG

Currently, hexo handles hexo.config.external_link inside the filter. With config being cached there will be no need to run Object.assign() again and again.

How to test

git clone -b optimize-external-filter 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.

[yoshinorin]

LGTM :)

BTW, we can extract this code into another common function :)

[SukkaW]

@yoshinorin

IMHO, we have no necessary to do that.
We already split isExternalLink() into hexo-util. The rest of the code is very specific and no need to split into util.