New plugin hooks: extra_css_urls() and extra_js_urls()

Closes #214

tests/fixtures.py

@@ -113,6 +113,19 @@ def prepare_connection(conn):
         "select convert_units(100, 'm', 'ft');"
         return (amount * ureg(from_)).to(to_).to_tuple()[0]
     conn.create_function('convert_units', 3, convert_units)
+
+
+@hookimpl
+def extra_css_urls():
+    return ['https://example.com/app.css']
+
+
+@hookimpl
+def extra_js_urls():
+    return [{
+        'url': 'https://example.com/app.js',
+        'sri': 'SRIHASH',
+    }]
 '''
 
 TABLES = '''