Dasher

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* Use cookiecutter to create a proper project structure.
* Refactor core functionality into ``dasher.Api``.
* Combine widget factory and template logic into unified layout implementation.
* Fix resizing bug when switching tabs by using callback-based tab switching.
* Add support of fully custom widgets.
* Add documentation.
* Add more examples.

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* Add ``_labels`` argument to the ``callback`` decorator to enable customization of
widget labels.

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* Add ``credits`` argument to DasherStandardTemplate to toggle whether to show credits
in the navbar.
* Update docstrings and documentation.
* Add margin to navbar.