added clone method to GoTweenConfig

`GoTweenConfig.clone()` lets you build on a `GoTweenConfig` instance without mutating it.

```csharp
var config = new GoTweenConfig().scale( 3 ).setIterations( 3 );
Go.to(someTransform, 1f, config.clone().position( Vector3.one );
// doesn't translate someOtherTransform
Go.to(someOtherTransform, 1f, config.clone().rotation( Quaternion.identity );
```