Add manual init method

[webmonch]

In current implementation in Expo projects when you use plugin to init the SDK - it inits at the very start of the app, even before the App.tsx is loaded. As a result if you try to hide intercom with hideIntercom or setInAppMessageVisibility('GONE') - messages are still visible for a second or so as SDK inits prior to that.

The only way to avoid it is to manually init plugin in a separate native plugin, which is not optimal, especially if you don't have native plugins in the project and need to add a new one just for that.

I added initialize() method and an option to disable auto init via plugin.

[robwalkerco]

@jasonpraful It would be great to get this, or a similar option, merged and released. I have the same issue with expo, where I only want to init Intercom once the we have dismissed the splash screen after some initial app bootstrapping, and currently, an in-app messages get displayed while the splash screen is still displayed.
It looks like we many need to use patch-package or a custom native module, to implement the desired behaviour at the moment,