Facilitate IOS entitlements

[ythomop]

The inspiration is Expo's app.json which lets us grant various permissions to our app using Entitlements. An example use case is when we want to read secret values in our widgets from the Keychain services stored by our app, e.g. an access token.

I opted to use plist.js so as not to reinvent the wheel.

[gitn00b1337]

I like this. I'll get some time next week to give it a test and merge asap after that if alls good. Thanks