This changes the renderer class to store the controller and defaults as
an instance variable rather than allocating a new class. You can create
a new renderer with an new env by calling `Renderer#new` or use new
defaults by calling `Renderer#with_defaults` and saving the return value
somewhere.
Also I want to keep the `env` private since I would like to change the
keys in the future. This commit only translates particular keys that
the user requested.