refactor(external_link): optimize regex pattern

[SukkaW]

What does it do?

Optimize the regexp used in external_link filter. The new regexp will avoid edge cases like a prefixed tag <article>, and it is faster (benchmark shows it is 18% faster).

How to test

git clone -b external_link_regex 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 remained the same at 97.128% when pulling 8fc15f9 on SukkaW:external_link_regex into 5fba0a0 on hexojs:master.