Adds feature toggles pulled from the environment

This is maybe a bit janky, but I think the idea of having environment-based feature toggles will be really helpful so feedback is appreciated!

The idea is that an environment variable defined as `FEATURE_TOGGLE_XXX` becomes available to jinja templates (rendered server- or client-side) as `{{ featureToggles.XXX }}`.

Values for the environment variable can be `true` or `false`, and templates can handle the `undefined` case as they want.

Whatchoo think @brianloveswords, @toolness, @andrewhayward?!