We'll want to pass in arguments to render (i.e., width),
so using __str__ is not going to be appropriate.

a) don't use format, use __str__, since nothing calls
   format directly
b) don't declare interest in keys; just don't compute
   rendered value in update method, only when actually
   rendering (in __str__)
c) don't pass in the available width to update, since
   that is only relevant during rendering (this will
   need to be fixed later, currently we don't get width
   at all)

Add a format() method that actually do the heavy duty lifting,
such as string formatting, which is only needed during actual
output.

Having to check it is costly.

but update is called, do not crash.