Crop-resistant / ImageMultiHash support

[bethylamine]

https://github.com/JohannesBuchner/imagehash/blob/dee4c95082641300aa7f30c0b97ee9b11a491fee/imagehash/__init__.py#L588

https://github.com/JohannesBuchner/imagehash/blob/dee4c95082641300aa7f30c0b97ee9b11a491fee/imagehash/__init__.py#L448

Support for hashing image segments will be the most straightforward way of handing screenshots-of-image matching.

[simon987]

the watershed segmentation bit will suck really bad to implement efficiently in javascript but I'll give it a try!

looks very promising according to the paper

[simon987]

@bethylamine I added cropResistantHash, hopefully this is useful for screenshot-of-images!

In the same commit I micro-optimised the phash function (~60% faster on my browser) so you should update even if it turns out that cropResistantHash doesn't work great for the extension :)