fix(excerpt): stricter regex

[curbengh]

What does it do?

The current regex to detect the excerpt tag <!-- more --> is too lenient, it matches <!--- more ---> and even

<!--
more
-->

This PR restricts it to two dashes only and zero or one space.

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.

[curbengh]

I label it BC in case users are not using the proper syntax.

---

Removed the label, the docs has always mention the syntax is <!-- more -->.

[coveralls]

Coverage remained the same at 98.237% when pulling de63ab9 on curbengh:rexcerpt into 49ff2b2 on hexojs:master.