The beginning of context hooks.

Not the working solution, but getting there conceptually.  Basically
we'll have a key with the view and the template as a tuple which is
the context hook that anyone can attach to.

However, some changes have still to be made:
 - The unit test doesn't work yet and contains a set_trace ;)
 - We'll probably switch the "view" component from being the callable
   to the "urlgen"'able name per Elrond's suggestion
 - Found a bug in unit tests related to running custom apps for
   different configs... hm.  I need to fix this!

Nonetheless, making progress.

This commit sponsored by... wait a minute... Christopher Webber?!

mediagoblin/tests/test_pluginapi.py

@@ -25,6 +25,7 @@ from mediagoblin import mg_globals
 from mediagoblin.init.plugins import setup_plugins
 from mediagoblin.init.config import read_mediagoblin_config
 from mediagoblin.tools import pluginapi
+from mediagoblin.tests.tools import get_app
 
 
 def with_cleanup(*modules_to_delete):
@@ -323,3 +324,14 @@ def test_plugin_config():
     # the callables thing shouldn't really have anything though.
     assert len(config['plugins'][
         'mediagoblin.tests.testplugins.callables1']) == 0
+
+
+@pytest.fixture()
+def context_modified_app(request):
+    get_app(
+        request,
+        mgoblin_config=pkg_resources.resource_filename(
+            'mediagoblin.tests', 'appconfig_context_modified.ini'))
+
+def test_modify_context(context_modified_app):
+    pytest.set_trace()