Preserves the old behaviour of not wrapping everything in p tags,
but also returns valid markup if the resulting markdown contains
multiple paragraphs (previously it stripped the <p> from the start
and the </p> from the end, leaving closing and opening paragraph tags
in the middle of the returned markup).
Also turn on the 'xhtml' option so marked uses self-closing tags
for br's, hr's and so forth.
ca240b0b85