- Clean up README formatting.
- Move all configuration documentation from functions into the README.
- Add missing documentation for a bunch of other options.
Fixes#337
Why:
1. `$_` is marked as deprecated
https://fishshell.com/docs/current/language.html#envvar-_
2. `$_` contains wrong process name in cases like this one:
```fish
sleep 1000 # `sleep` is the process, now press Ctrl-Z to suspend
fg # unsuspend it, now `fg` is shown as the process
```
`status current-command` will show `sleep` instead, which makes more
sense.
