Add the ability to implement your own SizeSelector

[iAlex97]

If you'd like to use other resolution than the provided biggestSize() and smallestSize() you can't or at least it's not documented. I'd like to do some JNI image processing and for that I need to find a sweet spot between the max and min resolutin.

[dmitry-zaitsev]

biggsetSize() and smallestSize() are actually returning an implementation of SelectorFunction. That is the interface which provides you with a list of available items (sizes in this case) and asks you to pick one of them.

SelectorFunction is public, so you can just provide your own implementation where you would normally call biggestSize().

Here are some examples of implementation:
https://github.com/Fotoapparat/Fotoapparat/blob/master/fotoapparat/src/main/java/io/fotoapparat/parameter/selector/SizeSelectors.java