not exactely sure what you're asking for - but I would
a) store the input text in 'markdown' format - quite simple, can be produced by any simple editor
b) have a markdown parser to parse the markdown formatted document
c) feed the output from the markdown parser into an rtf document builder - maybe something like
RTF Document Constructor Library[
^]
d) the result can then be displayed in an rtf textbox
'g'