Add mc/insert-letters function and test cases

* README.md: Add to readme, under special section.
 * features/insert-letters.feature: Add scenarios.
 * features/step-definitions/multiple-cursors-steps.el: Add call.
 * features/support/env.el: Add keybinding for insert-letters to H-3.
 * mc-separate-operations.el: Add function and helper functions.
 * multiple-cursors-core.el: Add insert-letters to functions that run once.
 * multiple-cursors.el: Add to readme.

mc-separate-operations.el

@@ -53,6 +53,31 @@
                             (mc/cursor-end cursor)) strings))))
     (nreverse strings)))
 
+;;;###autoload
+(defun mc/insert-letters (arg)
+  "Insert increasing letters for each cursor, starting at 0 or ARG.
+     Where letter[0]=a letter[2]=c letter[26]=aa"
+  (interactive "P")
+  (setq mc--insert-letters-number (or arg 0))
+  (mc/for-each-cursor-ordered
+   (mc/execute-command-for-fake-cursor 'mc--insert-letter-and-increase cursor)))
+
+(defun mc--number-to-letters (number)
+  (let ((letter
+	 (char-to-string
+	  (nth (mod number 26) '(?a ?b ?c ?d ?e ?f ?g ?h ?i ?j ?k ?l ?m ?n ?o ?p ?q ?r ?s ?t ?u ?v ?w ?x ?y ?z))))
+	(number2 (/ number 26)))
+    (if (> number2 0)
+	(concat (mc--number-to-letters (- number2 1)) letter)
+      letter)))
+
+(defvar mc--insert-letters-number 0)
+
+(defun mc--insert-letter-and-increase ()
+  (interactive)
+  (insert (mc--number-to-letters mc--insert-letters-number))
+  (setq mc--insert-letters-number (1+ mc--insert-letters-number)))
+
 (defvar mc--strings-to-replace nil)
 
 (defun mc--replace-region-strings-1 ()
@@ -94,13 +119,13 @@ highest colum number (the rightest).
 Might not behave as intended if more than one cursors are on the same line."
   (interactive "c")
   (let ((rightest-column (current-column)))
-    (mc/execute-command-for-all-cursors 
+    (mc/execute-command-for-all-cursors
      (lambda () "get the rightest cursor"
        (interactive)
        (setq rightest-column (max (current-column) rightest-column))
        ))
-    (mc/execute-command-for-all-cursors 
-     (lambda () 
+    (mc/execute-command-for-all-cursors
+     (lambda ()
        (interactive)
        (let ((missing-spaces (- rightest-column (current-column))))
 	 (save-excursion (insert (make-string missing-spaces character)))