Optional leading semicolons support

[alandipert]

via @micha:

(foo
  ;;{{
  ;; some stuff here
  ;; and whatntot
  ;; but the important
  ;; thing is the leading
  ;; semicolons because
  ;; that means you don't
  ;; need to balance parens
  ;; in here
  ;;}}
  bar baz)