Adding ability to (un/)subscribe to topics at firebase

[markosko]

Tested and works.
Have done only for android as i don't have Mac so can't test iOS but it's almost same way at iOS part.

[rcluan]

Hi, @markosko . In your unsubscribe method shouldn't be unsubscribeFromTopic() instead of unsubscribeToTopic() when using com.google.firebase.messaging.FirebaseMessaging.getInstance()?

[markosko]

Quite possible as i copied some things between lines :D i will change it :)

[rcluan]

I've tested and it's working like a charm both subscribe and unsubscribe methods. =D well done

[EddyVerbruggen]

Cool! Looks good to me as well. Let's merge this, or do you want to add a bit of documentation first? I can do that if you don't feel like it ;)

[markosko]

Dont mind to do some docs, but can someone later pull up ios side so that we have both platforms in sync ? :) tommorow I can add docs

[EddyVerbruggen]

Sweet, I'll take care of iOS as I'm planning on doing some maintenance work for this plugin in the next few days anyway.

[markosko]

and add it to messaging docs part right ?

[markosko]

and one more thing @EddyVerbruggen should I add info to docs about only android will be temporary available or no need for that ?

[EddyVerbruggen]

@markosko I'll look at it now and take care of any docs changes. Thanks!!