Configurator

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- Allow the list of values considered false by :meth:`if_supplied` to be specified.

- :meth:`if_supplied` no longer considers ``False`` to be false, as when present, that's
often an explicitly provided value.

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- :class:`Config` instances can now be pickled.

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- :func:`value` has been added to allow literal values to be used in the left
side of mappings passed to :meth:`Config.merge`.

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- Configuration values my now be set using attribute or item setting on
:class:`~.node.ConfigNode` instances.

- :meth:`~.node.ConfigNode.node` can be used to obtain or create a
:class:`~.node.ConfigNode` from a dotted path and will give you a node even
for a value of a :class:`dict` or item in a :class:`list`.

- :class:`~.node.ConfigNode` instances now have a :meth:`~.node.ConfigNode.set`
method that can be used to replace the value of that part of the configuration,
regardless of whether it is a container, list item or dictionary value.

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- Performance improvements when import parsers.

- Removed the ability to provide new parsers using `pkg_resources`
entry points.

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- Add :meth:`Config.from_env` class method to help with extacting
large numbers of environment variables into configuration.