Compatible with iOS 9
Message Markdown is a tweak that edits the iMessage app so that message bubbles now support basic markdown.
italics = _italic text_
bold = **bold text**
underlining = __underlined text__
code = `code text`
strikethrough = ~~strikethrough~~
coloured text = {#HEX|text}, HEX being a hex colour
coloured background text = {bg#HEX|text}
text spacing = {k4.5|widely spaced text}, k4.5 can be any number
See this reddit thread for info.
Configure options from Settings.