Allow the cropped file to be automatically written to a file

[vanniktech]

It would be really nice if the ImageCropper.cropSrc function were to be extended with a possibility to write the cropped image to the file system. This is kind of the counterpart of #24

Regarding writing a file either kotlinxio or okio could be used. Both are multiplatform. Right now the API is kinda cumbersome. I'm getting a ImageBitmap back from which I need to read all the pixels manually and write it to a file; or am I overseeing something?

[tamimattafi]

@vanniktech I agree that the api is limited to Bitmap right now, and it's kinda frustrating. But I believe after #24, we can add more save destinations, since we now have more data sources.