File Import

- Implemented import from task 1.4.3, 1.5.0, 1.6.0, plain text,
  todo.sh and task command line files.
- Implemented unit tests to cover the above.

html/task.html

@@ -121,6 +121,8 @@
                       export files from versions 1.4.3 and earlier, versions 1.5.0 and later,
                       todo.sh 2.x, CSV, plain text and task command line.  See online docs for
                       full details.
+                  <li>Export was including 'id' in the column header even though it was not
+                      included in the data.
                 </ul>
 
                 <p>