Added support for minimal Markdown "autolinks"

[KABoissonneault]

Sometimes, a URL is used both as text and link. ex: [https://example.com](https://example.com)

The credit section in my "About" markdown has quite a few of them. The duplication is annoying to do, and makes the Markdown source less readable.

This PR adds support for simple automatic links ("autolinks"). Many Markdown hosts (like Github) simply parse text to recognize URLs, but that would be too complicated for this project. Rather, we use the angle bracket syntax.

Source: https://www.markdownguide.org/basic-syntax/#urls-and-email-addresses

ex: <https://example.com>

This feature has the same restrictions as normal links (no headers, no emphasis), plus no image support (not sure whether it's normally supported or not, but I figure not).

I've also fixed an "unused variable" warning.

[juliettef]

Thanks we'll take a look.