BTW even WhatsApp has some basic kinda-markdown support: ```text``` becomes monospace, ~text~ means strikethrough, text is italics and text is bold. They also show the marks (but only in grey) and format it while typing, like @adbenitez described, but when it is received the marks are hidden.
Not saying we should go this way (e.g. it sounds like lots of ui work on each platform, more that just showing some HTML from the core), I just wanted to mention it.