Change: Load org only when needed

Closes #120.

README.org

@@ -136,6 +136,7 @@ Helm and Ivy are also supported.  Note that the =helm= and =ivy= packages are no
 
 *Changed*
 +  Option =magit-todos-exclude-globs= now excludes the `.git/` directory by default.  (Thanks to [[https://github.com/Amorymeltzer][Amorymeltzer]].)
++  Library ~org~ is no longer loaded automatically, but only when needed.  (This can reduce load time, especially if the user's Org configuration is complex.)  ([[https://github.com/alphapapa/magit-todos/issues/120][#120]].  Thanks to [[https://github.com/meedstrom][Martin Edström]] and [[https://github.com/jsigman][Johnny Sigman]] for suggesting.)
 
 *Internal*
 +  Define jumper keys using a Transient suffix.

magit-todos.el

@@ -64,7 +64,6 @@
 ;;;; Requirements
 
 (require 'cl-lib)
-(require 'org)
 (require 'grep)
 (require 'seq)
 
@@ -1044,6 +1043,7 @@ if the process's buffer has already been deleted."
 
 (defun magit-todos--format-org (item)
   "Return ITEM formatted as from an Org file."
+  (require 'org)
   (magit-todos--fontify-like-in-org-mode
    (concat (magit-todos-item-org-level item) " "
            (magit-todos-item-keyword item) " "