Don't fill comments. Affects #141
Refactor and simplify sx-question-mode--skip-FOO functions.

They no longer need to worry about restoring point. Just move point to
its destination and return non-nil if it worked.
Don't fill headlines
Reimplement reference not-filling
Reorganize functions.
Fix return values
Merge pull request #165 from vermiculus/better-filling

Better filling

Ignore HTML comments when filling body text and do not attempt to fill headlines.