Cache images

Closes #40
Various tweaks for #40
Use unicode arrows
Set the `progressive` to true by default and level to 7

This is now feasible since we cache the images, so it will only be run once.
Update deps
Update readme about caching of images
Switch to Chalk for colors

https://github.com/sindresorhus/chalk
Rename test file
Use `files` property in package.json
Limit async operations to the number of CPUs