Add a context for certain nodes, similar to the builtin module ast

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: PCManticore)

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There are a couple of nodes in the builtin module ast which provide a .ctx member, useful for determining in what context these nodes occurred, with types such as Store, for instance for x[1] = 2, Del for del x[1] and Load for the rest of them. Might be useful for pylint for disambiguating between a __getitem__ call or a __setitem__ call.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/267