Provide non -fn API

[lgrapenthin]

The -fn API is not very idiomatic and detains me from trying this lib. Skimming over the code, it does not appear to serve any particular purpose like pre-compiling anything. Instead it creates overhead of lambda creation/invocation at runtime.

It would be great if you could add a regular API where the image object is the first argument in every function.

[josephwilk]

I'm flexible to new apis. It was fun at the time but I agree the fn style is purely aesthetic.
If you wish to submit a pull request with your suggestion I would happily merge.

Thanks,
Joseph Wilk