- the context exists for the lifecycle of one card's render,
and caches calls to things like .card() to avoid add-ons needing to
do their own cache management.
- add-ons can optionally add extra data to the context if they need
it across multiple filters
- removed card_will_render. the legacy hook is still available for
now
- card_did_render is now called only once, with both front and back
text
This allows us to add a docstring to .append() so users can see
the names of the arguments that are being passed, and means we
don't have to remember to prepend run_ when calling a hook.
