Add support for custom HealthCheckResultHistoryItem models

[jackwh]

As discussed in #16 — this PR implements a model config parameter, for use by the EloquentHealthResultStore class.

I followed the general conventions used by http://github.com/spatie/laravel-activitylog for setting custom models — hopefully that's along the right lines this time.

This PR is backwards-compatible.

Thanks! 🙌

[freekmurze]

Thanks! I think there could be an issue when you want to use multiple stores, but I guess not too many people will do that.

[jackwh]

Thanks for your feedback 😊 Just made the changes, and PhpStan seems happy now — let me know if you'd like any other tweaks. 👌

[freekmurze]

Thanks!