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 />