Fill block comment paragraphs correctly

Makes `fill-paragraph' work with block comments like:

    func = ->
       ###
       Lorem ipsum dolor sit amet, consectetur adipiscing elit.
       Donec ut tellus et felis vulputate tincidunt.
       ###

Before, `fill-paragraph' would move the "Lorem ..." up next
to the first "###", which destroys the comment formatting.
Merge pull request #162 from denisw/master

Fill block comment paragraphs correctly