perf(external_link): update regexp

[SukkaW]

What does it do?

isExternalLink() has a shorthand optimization for absolute link:

https://github.com/hexojs/hexo-util/blob/fe64888f28acaa77f26eab04e7d358c6e6029967/lib/is_external_link.js#L19

Then I think why can't we exclude relative link directly in the rATag to avoid extra isExternalLink() and RegExp#test? So here is the PR.

How to test

git clone -b BRANCH https://github.com/USER/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 98.242% when pulling b4beec3 on SukkaW:external-link-regexp into ec1e39f on hexojs:master.