Fill block comment paragraphs correctly

This pull request makes `fill-paragraph` work with block comments like:

    func = ->
       ###
       Lorem ipsum dolor sit amet, consectetur adipiscing elit.
       Donec ut tellus et felis vulputate tincidunt.
       ###

Currently, `fill-paragraph` moves the "Lorem ..." up next to the first `###`, which destroys the comment formatting. WIth these changes, "Lorem" will stay in the next line, thus preserving the formatting.