Ability to pass meta data along with push notifications

[Stalinko]

This PR replicates this one but without it's issues: #49
I'll just copy the description:

Pusher Beams allows passing meta data along with message parameters: https://pusher.com/docs/beams/guides/publishing-to-multiple-devices#adding-metadata-to-a-notification

I'm using that in my project and decided to format as a handy API in PusherMessage class.
Tests added.
Also added .idea folder into .gitignore. It's created by PHPStorm.

[Stalinko]

styleci again alerts something but I can't understand what it wants. Do I have to fix anything again?

[hypnocill]

@Stalinko I see that a ton of time has passed but any idea if this PR will ever be merged or we are supposed to patch the library with the diff ourselves?

[Stalinko]

@hypnocill for some reason the maintainer of this project doesn't react at my PRs, seems like he abandoned it.

@themsaid @freekmurze @atymic what's the state of this project? Would you have some time to accept the PRs?

[hypnocill]

@Stalinko I see, thanks for answering. How did you proceed, did you patch the library yourself, forked it or found another one?

[Stalinko]

@hypnocill you can use my fork (https://github.com/Stalinko/pusher-push-notifications/tree/meta-data) directly in composer until this PR is accepted.
This is how I do it.

In composer.json inside require write this:

    "laravel-notification-channels/pusher-push-notifications": "dev-meta-data",

and also add this block to the end of the file:

  "repositories": [
    {
      "url": "https://github.com/Stalinko/pusher-push-notifications",
      "type": "git"
    }
  ]

[hypnocill]

@Stalinko
Thank you so much! I'll consider that or I'll make a local patch or fork because there are two other things I may need - making the beams instance public and having the library install for laravel 6 since now it doesn't due to the requiring of laravel/events 7+, etc