Fix prebuild should skip prune

Cache dirs

Update README.md

Failure messaging

Friendly failures

test and patch for cache overriding checked-in node_modules

test and implementation for cacheDirectories (camelCase)

Makefile

fix(README): heroku buildpack:set not config:set

Update README.md

use @jkutner's new buildpack command

Adding buildpack art

config:set wasn't enough to change the buildpack if you already had one set. Changed it to heroku buildpack:set to match the docs here: https://devcenter.heroku.com/articles/buildpacks

allow users to cache their own directories