feat: support custom attributes for external links in markdown

[ulivz]

Summary

• Close: Feature request: make target=_blank rel=… for external links optional #186

This PR is to support following format to add custom attributes for ONLY external links, internal link will keep the old behaviors.

[link-text]((querystring)link)

example 1

• Input:

[custom domain]((target=_self&class=my-link)http://surge.sh/)

• Output:

<a href="http://surge.sh/" target="_self" rel="noopener noreferrer" class="my-link">custom domain</a>

example 2

[custom domain]((target=&rel=)http://surge.sh/)

It will be rendered as:

<a href="http://surge.sh/">custom domain</a>

example 3

[custom domain](http://surge.sh/)

It will be rendered as:

<a href="http://surge.sh/" target="_blank" rel="noopener noreferrer">surge</a>

So this PR is fully downward compatible.

cc @meteorlxy @ycmjason

[ulivz]

Will add doc later.

[ycmjason]

Just my opinion 😄

I understand that you want to make markdown links more flexible 👍, but I have a feeling that this makes things a little bit too complex? If I were to type the querystring, it is probably easier for me to type html:

<a href="https://github.com">hi</a>

I think this feature request is legitimate and in my opinion the best solution would be having a global config that switches all links. For special cases, just let the user type <a> tags.

What do you think?

[mathiasbynens]

I think my request in #186 is entirely orthogonal to this proposal.

To clarify, #186 is asking for a way to disable the automatic addition of target="_blank" rel="…" to external links. I don’t need fine-grained control over attributes on links. I just want some kind of configuration setting that I can toggle to stop these attributes from appearing, without having to change my Markdown source files.

[ulivz]

@ycmjason It's truly making things more complicated, I agree with your opinion 👍

@mathiasbynens Excuse me, I just misunderstood your proposal and thought that you might need fine-grained control 😅

Closing it and go #357 to continue.