Add support for automatic hard breaks

[rambip]

Some very popular markdown tools (I know about obsidian.md, but it is probably not the only one) have an option to toggle true hard breaks, meaning any return character in the markdown will produce a html
tag.

It would be really nice to have that option in markdown-rs too !

[wooorm]

I recently discover GFM works the same way, at the syntax-level, so I think that's an okay idea.

[alex35mil]

Came to create the same issue. There's a LineEnding enum, but apparently, it's not related to parsing options, and there's no way to parse line feed into mdast::Break.

[NyxCode]

Stumbled over this as well. As far as I can tell, this crate only parses hard breaks, ignoring soft breaks alltogether.
My quick & dirty fix for this is to do let md = md.lines().map(str::trim_end).join(" \n"); before rendering to HTML.