RFC: command composition

[Robert Collins]

This is a bit rambly, but I need to get it out of my buffers :P.

I'd like to be able to modify the input to commands from a plugin,
without fully decorating the command. Fully decorating is complex
because you have to keep up with option changes and so on; if I could
add an option (or options), and when they are given be called with the
option and arguments (dict, list) tuple, then return that to the next
handler, I could modify the input access to the parsed options for a
command without having to adjust the plugin simply because a new
unrelated option was added.

On the down side, this won't give any clue that a plugin *will* break
when an option changes - e.g. if instead of --foo, --bar is needed, the
plugin will just stop working rather than giving a compile or
type-checking error.

-Rob
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