Erp5.extension.sectionextender

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- pep8

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- Do not assume buildout::parts to be linebreak-separated. It could be
normalized to whitespace-separated values by some other code.

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- Improve the way to declare a section extender by having a single
option 'section-extender' and one section per line as its values.
Many thanks to Mustapha Benali for the review and suggestion.

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- initial release