Set Custom User Id via script tag attribute

[YassineElbouchaibi]

Hello! It would be nice to be able to set a custom user id directly via attributes of the script tag used to load rybbit/script.js. This would be nice mainly because since Rybbit V2, script.js's main function is now async. This means that identification can't be done via script.onLoad -> rybbit.identify(userId) as rybbit is not always globally defined yet and it causes a race condition. Being able to pass the customUserId as an attribute would solve the problem.

Proposal:

• Add customUserId to ScriptConfig
• Support data-custom-user-id in parseScriptConfig
• Change storedUserId in Tracker.loadUserId, to use this.config.customUserId and fallback to localStorage.getItem("rybbit-user-id")

If you are happy with the proposal above and the feature request I can go ahead and implement it. Let me know!