2.0.0

• Can also be used from the global environment, not only from packages.
• set_config_in() function, to allow custom APIs. This means that
  packages does not have to use set_config() and get_config(), but
  they can provide their own API.
• Fix a get_config() bug, for composite values only the first element
  was returned.
• Fix a bug when key was not set at all. In these cases fallback was
  ignored in get_config().