Add smartcrop.js to help with optimal cropping

[Munter]

https://github.com/jwagner/smartcrop.js/

[papandreou]

There's also https://github.com/lovell/attention -- and talk about adopting features from it directly in sharp: lovell/sharp#295

[papandreou]

Added support for resize=x,y&crop=entropy and resize=x,y&crop=attention using the sharp 0.16.0+ feature (lovell/sharp#295).

Released in express-processimage 7.3.0.

[papandreou]

I don't know how it compares to smartcrop.js (which would be harder to integrate), but it looks very promising :)