Plock

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- Re-add ``--no-venv`` option to allow skipping virtualenv creation.

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- Add ability to use Unified Installer download cache via ``--unified``.
[pigeonflight]
- Add ability to pass an extra extends file via ``--extends``.
[href]
- Remove ``--unstable``.
- Remove BUILDOUT_OPT

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- Fix bug in check_command try/except

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- Remove PLOCK_EXPERT
- Remove PLOCK_TIMEOUT
- Remove create_dirs
- Remove first_time
- Remove write_config
- Re-add create_venv

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- Change remote plone URL from https://raw.github.com/plock/pins/master/plone-4.3 to https://raw.github.com/plock/pins/master/plone-4-3

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- Try bin/command before command