Completions for CLI #1119

Following up on #1118, it would be great to have cross-shell completions for the CLI.

These could be statically autogenerated from the output of --help options.

Would be great to have

1. cross-shell completions (bash, zsh, fish, powershell)
2. help introspection where supported i.e.

pre-commit i<TAB>
install  (Install the pre-commit script)  install-hooks  (Install hook environments...)

3. complete for installed hooks (i.e.

pre-commit run check-<TAB>
check-added-large-files (Prevent giant files from being committed.)
check-ast   (Simply check whether the files parse as valid python.)
check-toml      (This hook checks toml files for parseable syntax.)

4. a command line option to install the completions (i.e. pre-commit install-completions --shell zsh).