[MERGE] Add a new hook Commands['extend_command'] for plugins that want to alter commands without overriding the entire command.

[Robert Collins]

This implements the extend_command hook component of the commands hook
design I put forward yesterday; mergable as-is I hope, though not
terribly useful until some commands have things you can extend on
them :). However, its conceptually seperate from doing an extension to a
specific command, and I don't need the rest of the design at this point
- thus this rather targeted branch.

-Rob

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