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This reverts commit 6eeb9aa5.

- Addresses issue #8 by making it explicit the use of environment
  variables. Don't look-up env variables outside the option parsing
  context.
- Try a mitigation for issue #2 (garbled output)
- Implement more formalized command-line options based on argparse
  (cannot use click at this stage)

[build] Add support for setting up BOB_DOCUMENTATION_SERVER for bob.extension-based Sphinx doc builds