avy.el (avy--read-candidates): Custom keys for deleting last read char

Add a defcustom for list of events that delete last read char, with
the default being '(8 127) which represents C-h and DEL.

Fixes #251

avy.el

@@ -248,6 +248,12 @@ Typically, these modes don't use the text representation."
   "In case there is only one candidate jumps directly to it."
   :type 'boolean)
 
+(defcustom avy-del-last-char-by '(8 127)
+  "List of event types, i.e. key presses, that delete the last
+character read.  The default represents `C-h' and `DEL'.  See
+`event-convert-list'."
+  :type 'list)
+
 (defvar avy-ring (make-ring 20)
   "Hold the window and point history.")
 
@@ -1882,8 +1888,9 @@ newline."
   :type 'boolean)
 
 (defun avy--read-candidates (&optional re-builder)
-  "Read as many chars as possible and return their occurrences.
+  "Read as many chars as possible and return their occurrences.
-At least one char must be read, and then repeatedly one next char
+At least one char must be read, and then repeatedly one next char
+may be read if it is entered before `avy-timeout-seconds'.  Any
 key defined in `avy-del-last-char-by' (by default `C-h' and `DEL')
 deletes the last char entered, and `RET' exits with the
 currently read string immediately instead of waiting for another
@@ -1917,7 +1924,7 @@ Otherwise, the whole regex is highlighted."
                ((= char 13)
                 (if avy-enter-times-out
                     (setq break t)
-                  (setq str (concat str (list ?\n)))))
+                  (setq str (concat str (list ?\n)))))
                ;; Handle C-h, DEL
                ((memq char avy-del-last-char-by)
                 (let ((l (length str)))