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

avy-jump.el

@@ -137,14 +137,19 @@ When nil, punctuation chars will not be matched.
   "Goto X.
 X is (POS . WND)
 POS is either a position or (BEG . END)."
-  (if (null x)
-      (message "zero candidates")
+  (cond ((null x)
+         (message "zero candidates"))
+
+        ;; ignore exit from `avy-handler-function'
+        ((eq x 'exit))
+
+        (t
          (select-window (cdr x))
          (let ((pt (car x)))
            (when (consp pt)
              (setq pt (car pt)))
            (unless (= pt (point)) (push-mark))
-      (goto-char pt))))
+           (goto-char pt)))))
 
 (defun avy--process (candidates overlay-fn)
   "Select one of CANDIDATES using `avy-read'.
@@ -474,7 +479,18 @@ The window scope is determined by `avy-all-windows' (ARG negates it)."
 The window scope is determined by `avy-all-windows' (ARG negates it)."
   (interactive "P")
   (avy--with-avy-keys avy-goto-line
-    (avy--goto (avy--line arg))))
+    (let ((avy-handler-function
+           (lambda (char)
+             (if (or (< char ?0)
+                     (> char ?9))
+                 (avy-handler-default char)
+               (let ((line (read-from-minibuffer
+                            "Goto line: " (string char))))
+                 (when line
+                   (goto-char (point-min))
+                   (forward-line (1- (string-to-number line)))
+                   (throw 'done 'exit)))))))
+      (avy--goto (avy--line arg)))))
 
 ;;;###autoload
 (defun avy-copy-line (arg)

avy.el

@@ -109,6 +109,14 @@ KEYS is the path from the root of `avy-tree' to LEAF."
           (funcall walker key (cddr br))
         (avy-traverse (cdr br) walker key)))))
 
+(defun avy-handler-default (char)
+  "The default hander for a bad CHAR."
+  (signal 'user-error (list "No such candidate" char))
+  (throw 'done nil))
+
+(defvar avy-handler-function 'avy-handler-default
+  "A function to call for a bad `read-char' in `avy-read'.")
+
 (defun avy-read (tree display-fn cleanup-fn)
   "Select a leaf from TREE using consecutive `read-char'.
 
@@ -127,8 +135,7 @@ multiple DISPLAY-FN invokations."
         (if (setq branch (assoc char tree))
             (if (eq (car (setq tree (cdr branch))) 'leaf)
                 (throw 'done (cdr tree)))
-          (signal 'user-error (list "No such candidate" char))
-          (throw 'done nil))))))
+          (funcall avy-handler-function char))))))
 
 (provide 'avy)