Recurring Tasks - new "weekdays" frequency

- Added support for "weekdays" as a recurrence frequency, which skips
  Saturday and Sunday, but is otherwise a daily recurrence.  Thanks
  to Chris Pride.

html/recur.html

@@ -116,6 +116,13 @@ recurrences of this same task? (y/n) y</code></pre>
                     <td class="table_d">daily, day, 1d, 2d ...</td>
                     <td class="table_d">Every day, or a number of days</td>
                   </tr>
+                  <tr>
+                    <td class="table_d">weekdays</td>
+                    <td class="table_d">
+                      Monday, Tuesday, Wednesday, Thursday and Friday,
+                      skipping weekend days
+                    </td>
+                  </tr>
                   <tr>
                     <td class="table_d">weekly, 1w, 2w ...</td>
                     <td class="table_d">Every week, or a number of weeks</td>