Filling comments and strings

fill-paragraph, fill-region, and auto filling in comments and strings, based on woolsweater's work.

• Recognizes some Markdown blocks.
• Handles multiline comments:

  /* aaa bbb ccc */
  ↓↑
  /*
    aaa
    bbb
    ccc
  */
  

• If swift-mode:fill-paragraph-entire-comment-or-string is non-nil, fill-paragraph fills the entire comment.
• If comment-fill-column is defined, it is used in comments rather than fill-column.