externalLinkRel not working if externalLinkTarget: '_blank' #1183

Bug Report

Re. #917 / #914, when externalLinkTarget: '_blank' and externalLinkRel: 'noopener', href should be:

<a href="https://outthere.com" target="_blank" rel="noopener">

but it's being generated as:

<a href="https://outthere.com" target="_blank">

Steps to reproduce

1. Add an external link to a markdown file:

   [Out There](https://outthere.com)
   

2. Add this do window.$docsify:

   externalLinkTarget:  '_blank',
   externalLinkRel: 'noopener'
   

What is current behaviour

Links are generated without rel="noopener". Example:

<a href="https://outthere.com" target="_blank">

What is the expected behaviour

Links should have rel="noopener". Example:

<a href="https://outthere.com" target="_blank" rel="noopener">

Other relevant information

• Bug does still occur when all/other plugins are disabled?

• Your OS: GNU Linux

• Node.js version: none

• npm/yarn version: none

• Browser version: Chromium 81.0.4044.138

• Docsify version: 4.11.3

• Docsify plugins: docsify-copy-code, external-script, search, zoom-image, prism-bash, prism-json, prism-markdown, docsify-tabs

Please create a reproducible sandbox

Mention the docsify version in which this bug was not present (if any)

Not tested in other versions.

PS: add to showcase

I've just published a brand new website of my company made entirely with docsify. I would appreciate to have it listed in the showcase. The entry might look like:

- [eDok](https://www.edok.com.br/) - A safe, full featured ECM (Electronic Content Management) for the paperless business.