add spec for settings setter
add support for `config.setting_name?`

This adds a ActiveModel like questionmark attr checker:

  inheritable_setting :foo, true
  config.foo? # => true

  inheritable_setting :foo, false
  config.foo? # => false

  inheritable_setting :foo, ""
  config.foo? # => false

  inheritable_setting :foo, "bar"
  config.foo? # => true