Man writer:  give more fine-grained control in template.

Now the `title`, `section`, `header`, and `footer` can all be set
individually in metadata.  The `description` variable has been
removed.

Quotes have been added so that spaces are allowed in the title.

If you have a title that begins

    COMMAND(1) footer here | header here

pandoc will parse it as before into a title, section, header, and
footer.  But you can also specify these elements explicitly.

Closes #885.