Illegal paragraph breaks in `emph` and `textbf` macro calls

[harpocrates]

The following is not valid LaTeX, yet it is something Haddock could generate (and does in the documentation for text):

\emph{This is an important line. \par This is another important line}

The \par messes this up. Taking Pandoc's approach, we could generate something like this instead:

\emph{This is an important line.} \par emph{This is another important line}

[harpocrates]

Fixed in ae23b4f.