Injection of unique identifier for all unique-like rules

[Propaganistas]

I'd expect that any unique-like rule would need some identifier injection, so why not make it more generic in code?

This allows for ease of use in conjunction with for example the unique_with validator.

[dwightwatson]

Thanks for taking the time to put this together. Unfortunately I don't feel this PR is a good fit at the moment.

The reason the OR clause is in there is because of #114. I'd be happy to support other core Laravel unique* validators, but I'm not comfortable supporting a third-party one. If things change upstream on their end it means we'll have to change to stay compatible. If you want to use unique_with and this package, you'll probably be best to override that method in your app.

[Propaganistas]

Ok I understand.

How about this? #154