fix(meta_generator): handle multi-line head tag correctly

[curbengh]

What does it do?

Fix #3777
BREAKING CHANGE: requires Node >= 8.10

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/dotAll
• https://node.green/#ES2018-features

How to test

git clone -b meta-gen-fix https://github.com/hexojs/hexo.git
cd hexo
npm install
npm test

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.267% when pulling 4f7373b on curbengh:meta-gen-fix into 5d94690 on hexojs:master.

[SukkaW]

Since this fix is not so emergency, we can include this in v4.1 along with other changes.

[curbengh]

I just cleared netlify cache of Hexo.io to include this fix.