Pytest-play

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- Documentation updated (add new pytest play plugins)

- Support default payloads for command providers. Useful
for HTTP authentication headers, common database settings

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- execute command accepts kwargs now

- execute command returns the command value now

- complete refactor of ``include`` provider (no
backwards compatibility)

- add ``play_json.get_file_contents`` and removed
``data_getter`` fixture (no backwards compatibility)

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- play engine now logs commands to be executed and errors

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- you are able to update variables when executing commands

- you can extend ``pytest-play`` with new pluggable commands coming
from third party packages thanks to setuptools entrypoints

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- no more open browser by default
pytest-play is a generic test engine and it could be used for non UI tests too.

So there is no need to open the browser for non UI tests (eg: API tests)

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- implement reusable steps (include scenario)

- minor documentation changes