fill while in a comment should behave as in text modes

[ts4z]

This fixes a bug in fill-paragraph that made adaptive fill not work
while in a comment (probably the only time it matters in yaml-mode).

This change does what lisp-mode does: first, try fill-comment-paragraph.
If fill-comment-paragraph does the fill, great; we're in a comment, and
the right thing happens. If it returns nil, it can't do the fill, and
we can fall back to the previous method.

Without this change, a paragraph break inside a comment, or other things
that adaptive fill would generally detect, are not respected.

[wasamasa]

Looks good.

One more thing though, there's been interest into getting this mode into GNU ELPA. Are you willing to submit copyright assignment to the FSF, assuming you haven't already? I'd hate to accept contributions that would make reaching this goal harder :|

[ts4z]

I don't want to claim any copyright on this. I ripped it off from lisp-mode anyway, because that worked the way I expected. But I would sign if needed.

[wasamasa]

It would be nice if you could just universally forfeit copyright, but that's not a thing yet. This is why the FSF requires signing a copyright assignment form for every non-trivial contributor (meaning more than five lines of diff).

[ts4z]

Ok. I can either sign what needs signing or make it a smaller diff.