Link attributes get overwritten #224

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Bug report

Some link attributes can't be modified via markdown-it plugins like markdown-it-attrs, such as target and rel, because they are hard-coded:

https://github.com/saberland/saber/blob/4d5f4e33a3380fa26d831cd3fa22da9d0af3a37b/packages/saber/lib/markdown/link-plugin.js#L11-L16

This may look more like a feature, but it is inconvenient if one wants to change the attributes to something else like rel="nofollow" (must use HTML instead).

Version

0.6.7

Steps to reproduce

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1. In saber-config.js: plugins: [{ resolve: 'markdown-it-attrs' }]
2. Put a external link in a markdown page: [GitHub](https://github.com/) { rel="nofollow" }

What is expected?

Output is one of the following results or likewise:

• (Overwrite) <a href="https://github.com/" rel="nofollow" target="_blank">GitHub</a>
• (Join) <a href="https://github.com/" rel="noopener noreferrer nofollow" target="_blank">GitHub</a>

What is actually happening?

Output is <a href="https://github.com/" rel="noopener noreferrer" target="_blank">GitHub</a> (not containing rel="nofollow").

Other relevant information

Same for code blocks, but since their attributes are seldom modified, this should be considered fine.

https://github.com/saberland/saber/blob/4d5f4e33a3380fa26d831cd3fa22da9d0af3a37b/packages/saber/lib/markdown/highlight-plugin.js#L96-L99

And somehow related #199 (need an alternate syntax).

• Result of running saber -v: cli.js/0.6.7 win32-x64 node-v12.3.1
• Browser version (optional):
• Is Saber a global or local install? Local
• Which package manager did you use for the install? Yarn
• Does this issue occur when all plugins are disabled? No (apparently)