JSON Parse error when accessing some global Settings
When trying to access some Sketch’s preferences using the `Settings` API, we get a JSON Parse error. Running this on the Run Script panel…

```
const sketch = require('sketch')
console.log(sketch.Settings.globalSettingForKey('colorForMeasurements'))
```

…gives us:

```
SyntaxError: JSON Parse error: Unrecognized token '<'
    at parse[native code] (/)
    at onRun (/Users/ale/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Untitled.sketchplugin:3:48)
Script executed in 0.052706s
```

The `Settings` implementation assumes all settings can be parsed as JSON, and that's not the case for the ones we store as binary blobs using `NSKeyedArchiver` (`MSColorForSnapsAndGuidesKey`, `MSColorForMeasurementsKey` and `MSFlowArrowDrawingColorKey`, apparently)