Add support for variable templating in configured env vars

[nat-n]

Closes #38, also addresses #33

• Major refactor of internal APIs for managing env vars
• Set POE_ACTIVE in PoeExecutor.init instead of every implementation
• Fix issue caused by dependency on pypi in executor tests

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Works for both global and task level env variables.

Example usage:

.. code-block:: toml

[tool.poe.tasks.serve]
serve.cmd = "flask run"
serve.env   = { FLASK_RUN_PORT = "${TF_VAR_service_port}" }