Argcomplete

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- Re-add split_line to API (419)

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Fix zsh default completion issues

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- Fix zsh autoload issues

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- Fully support zsh. Argcomplete now supports completion descriptions
and global completion in zsh.

- Clean up top level namespace.

- Documentation and test improvements.

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- Test infrastructure improvements

- Indicate that there is no support commitment for fish and tcsh shells

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- Documentation and test improvements