Adds new env.settings dictionary that is interpolated in settings.php

Also adds deprecated library to store old setting interpolations
Merge branch 'feature/settings-interpolation' of github.com:duellj/butter into duellj-feature/settings-interpolation

* 'feature/settings-interpolation' of github.com:duellj/butter:
  Adds new env.settings dictionary that is interpolated in settings.php
Fixes pep8 error, spelling, and renames `default_settings` to `legacy_settings`.
Merge branch 'duellj-feature/settings-interpolation'

* duellj-feature/settings-interpolation:
  Fixes pep8 error, spelling, and renames `default_settings` to `legacy_settings`.
  Adds new env.settings dictionary that is interpolated in settings.php