-------------------------

Feature: Change ``:show-docstring:`` to take an optional value which is
the attribute to pull docstring content from. This means you don't have
to mix programming documentation with user documentation--they can be in
different attributes.

Feature: Add ``RequiredConfigMixin.get_runtime_config()`` which returns
the runtime configuration for a component or tree of components. This
lets you print runtime configuration at startup, generate INI files,
etc.

Feature: Add ``ConfigObjEnv`` which lets you use an object for
configuration. This works with argparse's Namespace amongst other
things.

Feature: Improve configuration-related exceptions. With Python 3,
configuration errors all derive from ``ConfigurationError`` and have
helpful error messages that should make it clear what's wrong with the
configuration value. With Python 2, you can get other kinds of
Exceptions thrown depending on the parser used, but configuration error
messages should still be helpful.

Documentation fixes and updates.