Parse the Markdown document stored in the string 'text' of size 'size'. The renderer provides callbacks to be called during the parsing so the caller can render the document on the screen or convert the Markdown to another format.

Zero is returned on success. If a runtime error occurs (e.g. a memory fails), -1 is returned. If the processing is aborted due any callback returning non-zero, md_parse() the return value of the callback is returned.