Add C-c C-c "do the right thing" command for emacs.

When in an existing comment, replies; when not in a comment, it
creates a new top-level comment for the current line.
Maintain context on newline in emacs.

If in a header, open a new header line; if in a body line, open a new
body line; otherwise, just do the normal newline thing.
Fill individual paragraphs in a comment body on meta-q.

Instead of filling the entire paragraph, to allow for, e.g., code-like
formatting in a comment.