Add custom command support to `prise.spawn()`

  Currently `prise.spawn()` only accepts cwd and always spawns the user's shell (`$SHELL` or `/bin/sh`).

It would be useful to spawn a PTY with a custom command when customizing the UI or manipulating it programmatically.
```
  prise.spawn({ cmd = "htop" })
  -- or with arguments:
  prise.spawn({ cmd = { "vim", "file.txt" } })
```