[F-REQ] Add withCount same way as $itemWith and $collectionWith

[leon0399]

Is your feature request related to a problem? Please describe.
It's now unable to add counts for relations in API without overriding almost all functions in default Controller at laravel5-api-boilerplate, so it'll be nice to add such functions to this library

Describe the solution you'd like
Add a new variable and function to RestfulModel the same way, as $itemWith and $collectionWith: $itemWithCount and $collectionWithCount

Describe alternatives you've considered
IMHO, there is no other alternatives

Additional context
@specialtactics let me know, if you like this feature, and I'll start working on PR

[specialtactics]

Hi @leon0399 - thanks for your interest.

I've never used that before, but after doing some research, I think it makes sense to add it. If you would like to do a PR to add it, then that's even better 😄

[leon0399]

Sorry, I'm still working on main project with your template 🤣

I'll contribute asap, when finish the project 😄