Add a before_fork callback called before forking new molds and new workers

[casperisfine]

Fix: #76

FYI: @skirushkin

[skirushkin]

I've checked this callback and it works perfect! Thank you so much!

Also I believe you need to fix docs here https://github.com/Shopify/pitchfork/blob/master/docs/FORK_SAFETY.md

And move Sequel::DATABASES.each(&:disconnect) to correct callback