When rendering arbitrary templates, it is helpful to not overwrite `env` keys with nil if they don't match any found in the `RACK_KEY_TRANSLATION`
This allows the developer to set the environment to exactly what is needed for rendering.
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.
