prefix i18nliner-specific stuff more appropriately, fixes #8

ActiveModel's `i18n_scope` is something different, and not easily
compatible with how ours is intended to be used. Rename it, as well as
some options to avoid that and future conflicts
pass along the :scope option to Scope#normalize_key

vanilla i18nliner doesn't use this, but this way libraries/apps that
extend it can apply different scoping logic when a :scope is set
v0.0.9