New method for adding meta-data to push messages

[Stalinko]

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]

I see the style checker didn't like !empty() construction without space after exclamation (who writes a space after exclamation? 😱) .
If necessary I can fix. How to make it better? Make another commit and create a new PR or how?

[andreaselia]

@Stalinko I'd probably recommend fixing the space as the entire repo follows the styleci set rules.

Also, the conflict will likely need fixing.

[atymic]

LGTM. I'll fix the style issues.

[atymic]

Could you resolve the conflicts please?

[Stalinko]

I've resolved the conflicts.
Sorry I'm not familiar with the process of contributing to Open Source. Any advice how to finalize it properly would be appreciated.