Modify `at-full' and `de-bruijn' overlays to color depth

* avy.el (avy-lead-face-2): New face.
(avy-lead-faces): New defconst. Each element will be used to color the
corresponding depth.
(avy--overlay-at-full): Use `avy-lead-faces' to represent the current
decision depth. For `de-bruijn', prepend the old stuff, so that nothing
moves.

Fixes #53

avy.el

@@ -142,6 +142,10 @@ Typically, these modes don't use the text representation.")
     '((t (:foreground "white" :background "gray")))
   "Face used for matched leading chars.")
 
+(defface avy-lead-face-2
+    '((t (:foreground "white" :background "#f86bf3")))
+  "Face used for leading chars.")
+
 (defface avy-lead-face
   '((t (:foreground "white" :background "#e52b50")))
   "Face used for the leading chars.")
@@ -150,6 +154,14 @@ Typically, these modes don't use the text representation.")
   '((t (:foreground "gray40")))
   "Face for whole window background during selection.")
 
+(defconst avy-lead-faces '(avy-lead-face
+                           avy-lead-face-0
+                           avy-lead-face-2
+                           avy-lead-face
+                           avy-lead-face-0
+                           avy-lead-face-2)
+  "Face sequence for `avy--overlay-at-full'.")
+
 ;;* Internals
 ;;** Tree
 (defmacro avy-multipop (lst n)
@@ -566,8 +578,16 @@ LEAF is normally ((BEG . END) . WND)."
                 (car leaf)))
          (wnd (cdr leaf))
          oov)
-    (when (or avy-highlight-first (> (length str) 1))
-      (set-text-properties 0 1 '(face avy-lead-face-0) str))
+    (dotimes (i len)
+      (set-text-properties (- len i 1) (- len i)
+                           `(face ,(nth i avy-lead-faces))
+                           str))
+    (when (eq avy-style 'de-bruijn)
+      (setq str (concat
+                 (propertize avy-current-path
+                             'face 'avy-lead-face-1)
+                 str))
+      (setq len (length str)))
     (with-selected-window wnd
       (save-excursion
         (goto-char beg)