Custom context params problem due to running plugin via cli 
Hello there!

__Task__: To send param from Sketch CLI when running the Sketch plugin.
__Issue__: the derived value of the JSON dictionary is of type object(expected string).

That's code of my plugin with commented output:
```javaScript
import { Document } from 'sketch/dom';

export default ({ documentPath }) => {
  const hardcodedUrl =          '/Users/alexander.gusev/QSE/infrastructure/repositories/qordoba-sketch-editor-renderer/sketches/zip.sketch';
  console.log(documentPath); // '/Users/alexander.gusev/QSE/infrastructure/repositories/qordoba-sketch-editor-renderer/sketches/zip.sketch'
  console.log(documentPath == hardcodedUrl); // true
  console.log(documentPath === hardcodedUrl); // false
  console.log(typeof documentPath); // 'object'
  
  console.log(typeof documentPath.toString()); // 'object'
  console.log(typeof documentPath.valueOf()); // Type error
  // TypeError: Type error
  //   at valueOf[native code] (/)
  //   at /Users/alexander.gusev/QSE/sketch-plugin/sketch-plugin/sketch-plugin.sketchplugin/Contents/Sketch/__fitText.js:123:43
  //   at __skpm_run (/Users/alexander.gusev/QSE/sketch-plugin/sketch-plugin/sketch-plugin.sketchplugin/Contents/Sketch/__fitText.js:151:19)
  //   at __skpm_run[native code] (/)% 
 
  console.log(Object.entries(documentPath)); // []
  console.log(Object.getOwnPropertyNames(documentPath)); // []

  const document = Document.open(documentPath); // Error
  // Error: An Obj-C exception occurred.
  //   at [native code] (/)
  //   at value (/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:82729)
  //   at /Users/alexander.gusev/QSE/sketch-plugin/sketch-plugin/sketch-plugin.sketchplugin/Contents/Sketch/__fitText.js:133:74
  //   at __skpm_run (/Users/alexander.gusev/QSE/sketch-plugin/sketch-plugin/sketch-plugin.sketchplugin/Contents/Sketch/__fitText.js:155:19)
  //   at __skpm_run[native code] (/)
  // nativeException: -[__NSCFString isFileURL]: unrecognized selector sent to instance 0x600002f78b80% 
}
```

That's a bash command to run the plugin:
```bash
path/to/sketchtool run path/to/sketchplugin identifier --context='{"documentPath": "/Users/alexander.gusev/QSE/infrastructure/repositories/qordoba-sketch-editor-renderer/sketches/zip.sketch"}'
```

The solution I found is to use a String object explicitly:
```javaScript
import { Document } from 'sketch/dom';

export default ({ documentPath }) => {
  const parsedDocumentPath = new String(documentPath).toString();
  const document = Document.open(parsedDocumentPath); // works fine
}
```

__Question__: Is it the most preferable way to cast context value to string?

PS: by the way, as I understand, you override Object.prototype props for some reason. If so, do you think that overriding default behaviour is a  good idea? 