Allow to break from `avy-goto-line' into `goto-line'

Allow to break from `avy-goto-line' into `goto-line'

* avy.el (avy-handler-default): New defun.
(avy-handler-function): New variable. Bind this temporarily to catch bad chars.
(avy-read): Forward to `avy-handler-default'.

* avy-jump.el (avy--goto): Consider the case of 'exit symbol being
  returned.
(avy-goto-line): Bind `avy-handler-function' to catch digits and call
`goto-line' in that case.

Fixes #29