It's impossible to enter a literal tab (\t) in the shell

It seems `shlex` has quite weird behaviour for escaped characters.
If you enter:

```
>>> command "a\tb"
```

The argument you get will be `atb`. Typing an actual literal tab character is ignored by the shell, so it's also not possible to enter a tab that way.

I don't see a very easy way to fix this, especially since `shlex` is not maintained anymore. Would you be okay with a PR that removes the dependency on `shlex` and uses `strconv.Unquote()` to handle escaping and quoting?