snippet(emacs-lisp): generalize thing-at-point

- Multiple keys aren't supported,
- The uuid is redundant with an already-unique name,
- The trailing comment is more frustrating than helpful.
- Add's `%` (placeholder for selected text),
- Removes contributor line because the snippet is sufficiently generic.

Author: hlissner

Diff for: emacs-lisp-mode/thing-at-point

@@ -1,8 +1,5 @@
 # -*- mode: snippet -*-
-# contributor: Xah Lee (XahLee.org)
 # name: thing-at-point
-# key: thing-at-point
-# uuid: thing-at-point
 # key: tap
 # --
-(thing-at-point '$0) ; symbol, list, sexp, defun, filename, url, email, word, sentence, whitespace, line, page ...
+(thing-at-point `%`$0)