Open up CropViewController for inhertiance.

[vanniktech]

References #270

My main goal is to have my own version of CropViewController which behaves a bit different as the actions are in the navigation bar. So all of the set up code can go into its own Controller. Also in my own ViewController I'd also link my features together and implement other interfaces that belong to the image editing section.

[guoyingtao]

If you want that Mantis creates an instance of a subclass CropViewController, maybe we can add an API like below

public func cropViewController<T: CropViewController>(image: UIImage,
                               config: Mantis.Config = Mantis.Config(),
                               cropToolbar: CropToolbarProtocol = CropToolbar(frame: .zero)) -> T {
    let cropViewController = T(config: config)
    cropViewController.cropView = buildCropView(with: image, and: config.cropViewConfig)
    cropViewController.cropToolbar = cropToolbar
    return cropViewController
}

[vanniktech]

Do you want it like this?

[guoyingtao]

Looks good.
I added a couple of more comments.

[guoyingtao]

@vanniktech
Would you mind also updating README and add an example to show how to create a subclass of CropViewController since we add a new API for it.

[vanniktech]

Like this?

[guoyingtao]

Great!
Thanks for adding this 👍

[vanniktech]

Pulled this already into my project and it has everything I need!

[guoyingtao]

Hi @vanniktech
Can you also address this issue , then I will approve the PR.

[vanniktech]

Oh yes! I didn't see that comment.