Py-css-styleguide

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* Add a Django view mixin ``StyleguideMixin`` to include in a view to use a manifest;
* Add a basic Django view ``StyleguideViewMixin`` based on ``TemplateView`` and
``StyleguideMixin``;
* Add a very basic Django project needed for testing;
* Add Django as a development environment requirement but the package is still not
dependent from Django in default environment;
* Updated documentation;

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* Rename some model methods:

* ``set_rule`` to ``_set_rule``;
* ``remove_rule`` to ``_remove_rule``;

* Implement model method ``from_dict`` to enable loading manifest directly from a
dictionnary in the same format than ``to_dict`` so it can be used from a JSON dump
made by ``to_json`` (after have been deserialized).
* Add more manifest reference validations;
* Add ``exceptions`` module for application exceptions;

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* Fix documentation typo issue, close 13;
* Add ``Manifest.to_dict()`` and make ``Manifest.to_json()`` using it, close 14;
* Add property option ``--excludes`` in meta reference rule to ignore some explicitely
defined reference names in automatic mode, close 10;
* Update Package structure to use more modern configurations;
* Drop support for Python 3.5;
* Add support for Python from 3.6 to 3.8;

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* Added some Sass functions to escape a value from quotes;
* Enabled quote escape on ``get-props-to-json`` and ``get-values-to-json`` functions to
avoid invalid JSON, close 9;

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* Pinned ``tinycss2`` version to ``>=1.0.2``
* Updated parser so double dashes for CSS variable are correctly supported now, close 8;
* Rewrite package to use setup.cfg and virtualenv (instead of Python-venv);

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* Enforce order on flat structure, close 6;
* Added ``splitter`` property for ``flat``, ``nested`` and ``list`` structure to be
able to use either white space separator or JSON list on values, close 7;
* Added JSON structure;
* Added new Sass helpers to build JSON list from Sass lists, map key names, values and
properties;
* Removed unused method ``ManifestSerializer.format_value``;