Add Markdown support for single line breaks and strikeout

Reformulation of https://github.com/vanilla/vanilla/pull/2221 per notes on Spoilers. This version is much more conservative since it assumes future improved spoiler support will be core.

* Correct Markdown class markup & documentation per Vanilla standards
* Add single line breaks via `Gdn_Format`
* Add strikeout support
This is wrong and caused a lot of extraneous breaks before. There is a logical place inside of the markdown code to do this.