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."""

flask/blueprints.py

@@ -185,16 +185,29 @@ class Blueprint(_PackageBoundObject):
             return f
         return decorator
 
-    def app_template_filter(self, name = None):
-        """Like :meth:`Flask.template_filter` but for a blueprint. The filter
-        is available for the entire application.
+    def app_template_filter(self, name=None):
+        """Register a custom template filter, available application wide.  Like
+        :meth:`Flask.template_filter` but for a blueprint.
+
+        :param name: the optional name of the filter, otherwise the
+                     function name will be used.
         """
         def decorator(f):
+            self.add_app_template_filter(f, name=name)
+            return f
+        return decorator
+
+    def add_app_template_filter(self, f, name=None):
+        """Register a custom template filter, available application wide.  Like
+        :meth:`Flask.add_template_filter` but for a blueprint.  Works exactly
+        like the :meth:`app_template_filter` decorator.
+
+        :param name: the optional name of the filter, otherwise the
+                     function name will be used.
+        """
         def register_template(state):
             state.app.jinja_env.filters[name or f.__name__] = f
         self.record_once(register_template)
-            return f
-        return decorator
 
     def before_request(self, f):
         """Like :meth:`Flask.before_request` but for a blueprint.  This function