* thingatpt.el: Add support for email addresses (`email').

* thingatpt.el: Add support for email addresses (`email').
  (thing-at-point, bounds-of-thing-at-point): Document `email' support.
  (thing-at-point-email-regexp): New variable.
  (`email'): Put `bounds-of-thing-at-point' and `thing-at-point'
  properties on this symbol, with lambda forms for values.