External links that do not follow the post `http(s)://` scheme

[JoelMon]

Hello everyone,

Are Markdown links that start with www. legal? If so, I propose allowing links beginning with www. also.

Code Snippet

https://github.com/getzola/zola/blob/195b6bdff070913129f310f4fe68b42b5c8ba0a6/components/markdown/src/markdown.rs#L133-L136

Test

https://github.com/getzola/zola/blob/195b6bdff070913129f310f4fe68b42b5c8ba0a6/components/markdown/src/markdown.rs#L597-L609