Shell commands (#547)

* 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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