Add method to store sensitive data
Security is always good to consider and the simple options available for storing data in a plugin can easily lead to vulnerabilities. 

Pulling from this [forum post](https://sketchplugins.com/d/893-storing-sensitive-data/2) it would be great to add API for setting and getting sensitive data:
```javascript
Settings.secureKey(key)
Settings.setSecureKey(key, value)
```

Best would probably be for this to link into keychain though I'm not sure how well cocoa script with interface with that.