Hashing the output

[basil-cow]

I think it's an important feature is to keep a hash of the input data and options somewhere in the output folder in order to do nothing in case nothing changed since the last time the packer ran so you can call sheep painlessly on your game startup.

[happenslol]

Similar to #26, if at all, I think this should be implemented in the CLI and not in the library itself, since existing asset pipelines are very likely to implement this feature. Apart from that though, I feel like this is a great idea!