feat(filter): use existing excerpt if possible

[ppoffice]

What does it do?

Skip excerpt generation for posts or pages if it is set in the front matter. This gives users the ability to write down whatever abstract they want instead of just showing begining part of the post.

For example:

title: An article about blah blah blah
excerpt: This article is all about blah blah blah blah blah blah
---
Actual markdown content...

will generate:

{
    excerpt: 'This article is all about blah blah blah blah blah blah',
    more: 'Actual markdown content...',
    content: 'Actual markdown content...'
}

How to test

git clone -b BRANCH https://github.com/ppoffice/hexo.git
cd hexo
npm install
npm test

Screenshots

N/A

Pull request tasks

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

[coveralls]

Coverage increased (+0.002%) to 97.15% when pulling c321663 on ppoffice:master into f2ddc99 on hexojs:master.

[coveralls]

Coverage increased (+0.002%) to 97.15% when pulling d819f18 on ppoffice:master into f2ddc99 on hexojs:master.

[curbengh]

I prefer this approach (using it myself) than <!--more-->, looks cleaner. <!--more--> does have the advantage of supporting formatting though.