load_template() plugin hook

Closes #1042

tests/plugins/my_plugin.py

@@ -308,3 +308,9 @@ def table_actions(datasette, database, table, actor):
             },
             {"href": datasette.urls.instance(), "label": "Table: {}".format(table)},
         ]
+
+
+@hookimpl
+def load_template(template, request):
+    if template == "show_json.html" and request.args.get("_special"):
+        return "<h1>Special show_json: {{ filename }}</h1>"