Render Markdown text to Html using one of the
options defined in the Option module.
Like toHtml, but constructs a table of contents.
Like toHtmlWithTOC, but constructs returns a record,
one field of which is the rendered document,
anther of which is the rendered table of contents.
Parse the input and render it to Html, e.g.,
toHtml ExtendedMath "Pythagoras said: $a^2 + b^2 c^2$."