Get all instances does not return symbols that are used as override
For a plugin that I am writing I need to check the master.id before importing a symbol. This is currently not possible and I have to import all symbols from a library before I can check the master ID. This issue has already been reported here: https://github.com/BohemianCoding/SketchAPI/issues/203 

One of the side effects is that I need to clean up the imported symbol masters that are unused. I was hoping that this would happen automaticly on save but it does not appear to do so.
What I currently do is the following:
```
document.getSymbols().forEach((symbolMaster) => {
    if (symbolMaster.getAllInstances().length == 0) {
        symbolMaster.sketchObject.removeFromParent();
    }
});
```

One of the unlucky side effects is that all the nested symbols are also unlinked.

![image](https://user-images.githubusercontent.com/9990026/41455756-8f02296c-707e-11e8-9df9-85b0e68d4792.png)

I could implement the following code:
https://github.com/Ashung/Automate-Sketch/blob/master/automate-sketch.sketchplugin/Contents/Sketch/Symbol/Remove_Unused_Symbols_New.cocoascript

![image](https://user-images.githubusercontent.com/9990026/41456716-a5b84058-7081-11e8-9a80-2f71079509f4.png)

But this means another loop over all my symbols and their overrides which I would like to prevent

Would it be possible to let get all instances also return instances that are used as override? Then I would not have this probem.
https://developer.sketchapp.com/reference/api/#get-all-the-instances   

