inject protocol RVMViewModelReacting for using in couple RVMViewModel

Please, review request to inject protocol

Recently I faced to using multiple ViewModels with one common protocol. And you know, I don't understand why not use protocol to avoid some writting like ```RVMViewModel <MyProtocol> *viewModel```. Because, in this case a ViewController doesn't may find anything from ```RVMViewModel```

for example:

```objective-c
@protocol MyProtocol <NSObject>

@interface MyClass1VM: RVMViewModel <MyProtocol>
@interface MyClass2VM: RVMViewModel <MyProtocol>
```
thus, we get next ...

```objective-c
@interface MyCustomVC : UIViewController
@property (nonatomic, strong) RVMViewModel <MyProtocol> *viewModel
@end
```

Perhaps, better to use in this case a protocol inheritance

```objective-c
@protocol MyProtocol <NSObject, RVMViewModelReacting>
```