Patch - Support tag exclusion filtering

- Now supports "-tag" in filters to filter out tasks that do not
  have the specified tag (thanks to Chris Pride).
- Added unit tests to prevent regression.
- Updated the filter docs to include examples.

html/filter.html

@@ -76,6 +76,13 @@
                 <p>
                   Lists only tasks with the "shopping" tag.
                 </p>
+
+                <code><pre>% task list +shopping -gift</pre></code>
+
+                <p>
+                  Lists tasks that have the "shopping" tag, but do not have the
+                  "gift" tag.
+                </p>
               </div>
 
               <br />

html/task.html

@@ -144,7 +144,10 @@
                       part of the description, despite what they otherwise might mean.
                   <li>Removed support for the obsolete task file format 1 (never released).
                   <li>Fixed bug that allowed blank annotations to be added (thanks to Bruce
-                      Dillahunty),
+                      Dillahunty).
+                  <li>Supports negative tag filters, so that (task list +foo -bar) now filters
+                      tasks that have the "foo" tag, but do not have the "bar" tag (thanks to
+                      Chris Pride).
                 </ul>
 
                 <p>