Consider adding an `s.Okay = ` to replace the final glyph with another glyph on `s.Stop()`

Hi,

I wonder if you could consider this addition. Whilst there is a `s.FinalMsg = `, it might be nice to also have an `s.Okay = `, so that on call to `s.Stop()`, the final spin 'glyph' is replaced by another 'glyph', e.g.,

```
s.Okay = "✅"
s.Start()
s.Stop()
```

`Doing something...⠧`

then finally, it's done and `s.Stop()` is called...

`Doing something...✅`

-=david=-

