Enhancement - duplicate command

- Added duplicate command that duplicates a task, and en passant
  applies and specified changes (thanks to David J Patrick).

html/advanced.html

@@ -120,6 +120,13 @@ ID Project Pri Due Active Age     Description
                   variable.
                 </p>
 
+                <strong>% task duplicate 1 /foo/bar/g +tag priority:H</strong>
+                <p>
+                  This duplicates task 1, then applies the modifications specified,
+                  which change all "foo" to "bar" in the description and annotations,
+                  adds the tag "tag", and sets the priority to "H".
+                </p>
+
                 <strong>% task delete &lt;id&gt;</strong>
                 <p>
                   There are two ways of getting rid of tasks - mark them as done, or