perf(external_link): drop cheerio and use regex

[SukkaW]

What does it do?

Drop cheerio in external_link() helper for better performance.

How to test

git clone -b drop-cheerio-for-external-link https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

	NodeJS 8	NodeJS 10	NodeJS 12
[email protected] (hexojs/hexo#master)	7.64	6.62	7.54
regex (This PR)	7.60	6.57	7.40

	NodeJS 8	NodeJS 10	NodeJS 12
[email protected] (hexojs/hexo#master)	6.55	5.53	6.15
regex (This PR)	6.48	5.42	6.10

	NodeJS 8	NodeJS 10	NodeJS 12
[email protected] (hexojs/hexo#master)	6.12	5.92	6.57
regex (This PR)	6.03	5.88	6.44

The performance increased by nearly 1%.

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.003%) to 97.152% when pulling 07d83c7 on SukkaW:drop-cheerio-for-external-link into 6f16116 on hexojs:master.

[segayuu]

LGTM!

[SukkaW]

@segayuu I am thinking is it worthy.

In this PR I drop cheerio, completely rewrite this part use regex and replace(), but only result in less than 1% performance increases.

[curbengh]

This PR drops cheerio (along with its deps) and replace it with even less LOC than before. The way I see it, this is equivalent to >99% reduction of LOC. In this case, I'm certain regex+replace can perform at least as fast as cheerio.

LOC reduction + no loss in performance = 💯% worth it.