Fixed #17085, #24783 -- Refactored template library registration.

* Converted the ``libraries`` and ``builtins`` globals of
  ``django.template.base`` into properties of the Engine class.
* Added a public API for explicit registration of libraries and builtins.

docs/ref/templates/api.txt

@@ -41,7 +41,7 @@ lower level APIs:
 Configuring an engine
 =====================
 
-.. class:: Engine([dirs][, app_dirs][, allowed_include_roots][, context_processors][, debug][, loaders][, string_if_invalid][, file_charset])
+.. class:: Engine([dirs][, app_dirs][, allowed_include_roots][, context_processors][, debug][, loaders][, string_if_invalid][, file_charset][, libraries][, builtins])
 
     .. versionadded:: 1.8
 
@@ -114,6 +114,34 @@ Configuring an engine
 
       It defaults to ``'utf-8'``.
 
+    * ``'libraries'``: A dictionary of labels and dotted Python paths of template
+      tag modules to register with the template engine. This is used to add new
+      libraries or provide alternate labels for existing ones. For example::
+
+          Engine(
+              libraries={
+                  'myapp_tags': 'path.to.myapp.tags',
+                  'admin.urls': 'django.contrib.admin.templatetags.admin_urls',
+              },
+          )
+
+      Libraries can be loaded by passing the corresponding dictionary key to
+      the :ttag:`{% load %}<load>` tag.
+
+    * ``'builtins'``: A list of dotted Python paths of template tag modules to
+      add to :doc:`built-ins </ref/templates/builtins>`. For example::
+
+          Engine(
+              builtins=['myapp.builtins'],
+          )
+
+      Tags and filters from built-in libraries can be used without first calling
+      the :ttag:`{% load %}<load>` tag.
+
+.. versionadded:: 1.9
+
+    The ``libraries`` and ``builtins`` arguments were added.
+
 .. staticmethod:: Engine.get_default()
 
     When a Django project configures one and only one