* helm-elisp.el (helm-thing-before-point): New optional arg.
(helm-bounds-of-thing-before-point): DRY, use `helm-thing-before-point'.
* helm-dabbrev.el (helm-dabbrev--collect): Start searching backward before target.
(helm-source-dabbrev): Fix string insertion in action (same behavior as lisp completion).
(helm-dabbrev): Use thing before point.