File-config

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- adding serialization and deserialization support for enums

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- adding ``encoder`` and ``decoder`` var kwargs for customizing how a specific var is serialized/deserialized
- adding support for `python-rapidjson <https://pypi.org/project/python-rapidjson/>`_ as json serializer

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- adding ``sort_keys`` support for ``json`` dumpers
- adding conditional ``validate`` boolean flag for ``load_<json,toml,yaml,etc...>`` config method (performs pre-validation of loaded dictionary)
- fixing typecasting of loaded content when var is missing in content, now sets var to None
- improved tests via a hypothesis dynmaic config instance builder
- removing support for `complex <https://docs.python.org/3.8/library/functions.htmlcomplex>`_ vars since no serializers support them

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- adding ``prefer`` keyword to use specific serialization handler
- adding ``inline_tables`` argument for ``toml`` handlers (takes a list of fnmatch patterns)
- adding support for `toml <https://github.com/uiri/toml>`_

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- fixing ``make_config`` not using any passed in ``file_config.var`` instances
- added ``indent`` dumping argument for ``JSONHandler``
- improved documentation in ``file_config.schema_builder``
- improved sphinx linking from ``getting-started.rst`` to generated autodocs

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- added better docstrings
- added better documentation in rtd
- fixed ``file_config._file_config._build`` for ``file_config.Regex`` types
- fixed ``file_config.utils.typecast`` for serializing to ``str`` instead of None