Add subkey option to connect

[theJian]

This PR added an option subkey to connectAdvanced.

Currently, the createProvider accepted the second optional argument subKey, It seems like has no real usage for now. Providing the custom subscription key only to Provider is not sufficient for using this feature, connect components need to know the custom subscription key to access the subscription corresponding to custom store. By adding a new option subKey, connect can be able to access the specific subscription.

• Add an option subKey to connectAdvanced
• Add tests to Provider
• Delete extra whitespaces / Identing lines in codebase.

[theJian]

Btw I would like to know if it's possible to add .editorconfig and .prettierrc.json to this project. For a consistent code style. I found those config files does exist in reactjs/redux. Thanks.

[timdorr]

I'd argue we can just get rid of subKey. It doesn't get you anything. You know the subscription key because it follows a specific pattern. It's just extra complexity without any benefits.

[theJian]

@timdorr That's a good point. I will send a pr to remove the subKey another day. We can close this one now.

[jimbolla]

Yeah I don't like subKey, the sub and the store are supposed to go together. I would've made them a single thing if there was a way to do it that was backwards compatible.