Plock

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- Document PLOCK_EXPERT environment variable to respect ``.buildout/default.cfg``.
- Bug fix: fix reference to ``args.add_on`` via ``--preserve``.

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- Fix "brown bag"

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- Make "secure": all Buildout configuration files (for Plone, Zope2, the ZTK, etc.) are included in plock; this eliminates the possibility of a MITM-attack via remote extends (now you just need to trust PyPI and dist.plone.org.)
- Renamed script: ``install-plone`` to ``plock``.
- Renamed arg: ``--install-addons`` to ``--add-on`` (to improve argparse-provided usage statement).

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- Bug fix: don't print "Wrote buildout.cfg" if buildout.cfg exists.

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- Added ``--write-config`` to write buildout.cfg and exit.

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- Bug fix: re-add -U