Enhancement - substitutions /from/to/g

- Added support for the "g" modifier to the substitution command,
  that replace every occurrence of "from" with "to", in the task
  description and any annotations.

html/task.html

@@ -145,6 +145,8 @@
                       date, because of an assumption of 365 days per year, which failed to
                       consider leap years (thanks to T. Charles Yun).
                   <li>Annotations can now be modified with the substitution commands /from/to/.
+                  <li>Substitutions can now be made global with /from/to/g and all occurrences
+                      of "from" will be replaced with "to".
                 </ul>
 
                 <p>