Issue title: header link in github flavored markdown mode. 
 Issue body: <!-- Please don't delete this template or we'll close your issue -->
<!-- Please use English language -->
<!-- Before creating an issue please make sure you are using the latest version of Docsify. -->
<!-- Please ask questions on StackOverflow: https://stackoverflow.com/questions/ask?tags=docsify -->

## Bug Report

#### Steps to reproduce

set 'gfm' attribute to true and make a header link that has a hyphen('-') in it.  

#### What is current behaviour

I have to write in this way

```md
# link to  'part1 - my title'   
[my header link](part1-my-title)
```

#### What is the expected behaviour

but when I make header link in github, I have to write in this way.

```md
# link to  'part1 - my title'   
[my header link](part1---my-title)
```

I think link text should be same as github because I set gfm mode to true.

[I reported a issue to markedjs first.](https://github.com/markedjs/marked/issues/1579)

they said new release of docsify could fix this issue.

markedjs was updated in version 4.10.2 but nothing changed. 

so I think it is docsify's issue.

#### Other relevant information

<!-- (Update "[ ]" to "[x]" to check a box) -->
- [x] Bug does still occur when all/other plugins are disabled?

- Your OS: MacOS Mojave 10.14.16
- Node.js version: 13.1.0
- npm/yarn version: npm 6.12.1
- Browser version: safari 13.0.4
- Docsify version: 4.10.2(docsify) 4.4.0(docsify-cli)
- Docsify plugins:
``` 
  <script src="//unpkg.com/prismjs/components/prism-bash.min.js"></script>
  <script src="//unpkg.com/prismjs/components/prism-c.js"></script>
  <script src="//unpkg.com/prismjs/components/prism-cpp.js"></script>
  <script src="//unpkg.com/prismjs/components/prism-java.js"></script>
  <script src="//unpkg.com/docsify-copy-code"></script>
  <script src="https://cdn.jsdelivr.net/npm/docsify-themeable@0"></script>
```


<!-- Love docsify? Please consider supporting our collective:
👉  https://opencollective.com/docsify/donate -->

 Comments: 
 Comment 0: We'll need to investigate this. 
 Comment 1: This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

{'source': 'GithubIssues', 'repo_name_with_owner': 'docsifyjs/docsify', 'issue_id': 550581445, 'repo_stars': 15932.0}