Support of multiple class names with Markdown helper

[paulhibbitts]

Bug Report

Steps to reproduce

1. Use Markdown helper to assign multiple classes to link or image, for example with Docsify v5 [Button Title](https://google.com ':class=button primary')

Current behaviour

Currently only the first class included in quotes is applied, and the others are ignored.

Expected behaviour

All classes contained in the quotes should be applied.

Other relevant information

• Docsify version:

• [x] Bug still occurs when all/other plugins are disabled?

• Docsify plugins (if the bug happens when plugins enabled, please try to isolate the issue):

Please create a reproducible sandbox

https://codesandbox.io/p/sandbox/winter-darkness-2nk8hp

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