* Implement shell interaction commands
* Use slice instead of iterator for shell invocation
* Default to `sh` instead of `$SHELL` for shell commands
* Enforce trailing comma in `commands` macro
* Use `|` register for shell commands
* Move shell config to `editor` and use in command
* Update shell command prompts
* Remove clone of shell config
* Change shell function names to match prompts
* Log stderr contents upon external command error
* Remove `unwrap` calls on potential common errors
`shell` will no longer panic if:
* The user-configured shell cannot be found
* The shell command does not output UTF-8
* Remove redundant `pipe` parameter
* Rename `ShellBehavior::None` to `Ignore`
* Display error when shell command is used and `shell = []`
* Document shell commands in `keymap.md`
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