In building https://github.com/simonw/datasette-atom it became clear that the callback function (which currently accepts just args, data and view_name) would also benefit from access to a mechanism to render templates and a datasette instance so it can execute SQL.

To maintain backwards compatibility with existing plugins, we can introspect the callback function to see if it wants those new arguments or not.

At a minimum I want to make datasette and ASGI scope available.