Add non-decorator template filter methods.

Suggested by @Poincare on GitHub, on @Reisen's pull request:
https://github.com/mitsuhiko/flask/pull/272

flask/app.py

@@ -1018,10 +1018,20 @@ class Flask(_PackageBoundObject):
                      function name will be used.
         """
         def decorator(f):
-            self.jinja_env.filters[name or f.__name__] = f
+            self.add_template_filter(f, name=name)
             return f
         return decorator
 
+    @setupmethod
+    def add_template_filter(self, f, name=None):
+        """Register a custom template filter.  Works exactly like the
+        :meth:`template_filter` decorator.
+
+        :param name: the optional name of the filter, otherwise the
+                     function name will be used.
+        """
+        self.jinja_env.filters[name or f.__name__] = f
+
     @setupmethod
     def before_request(self, f):
         """Registers a function to run before each request."""