Rust: Support commonly used markdown extensions

[AlexApps99]

These include:

• Task list items
• Strikethrough
• Autolinks

The reasoning for all of these extensions is that they all have wide use not just in GitHub, but in many applications of Markdown.

[joshtriplett]

I'm not sure if automatically creating http:// links from bare domain names is a good idea, so if there's an option to disable that specifically I'd suggest it. The rest all seems fine to me.

[GuillaumeGomez]

Just like @joshtriplett said for Autolinks. I'm also not sure to see the point of the task list one. However, adding strikethrough would be nice.

[AlexApps99]

Tasklist would be useful for things like this:

Currently implemented:

• [x] Implemented thing
• [x] Some other implemented thing
• [ ] Unimplemented thing
• [ ] Some other unimplemented thing

or possibly in the crate root

Supported features

• [x] Webhooks
• [x] Websockets
• [ ] Websites
• [ ] Other

[AlexApps99]

Another noteworthy extension to implement is Disallowed Raw HTML.
This would prevent the following tags from being used for safety reasons:

• <title>
• <textarea>
• <style>
• <xmp>
• <iframe>
• <noembed>
• <noframes>
• <script>
• <plaintext>

[jyn514]

Autolinks

We decided in https://github.com/rust-lang/rust/issues/77501 not to add this. Instead rustdoc will now suggest to turn raw links into automatic links (<http://example.com>).

So I think the only thing left is adding task list parsing.