Add 'daemonize' config

[rhannequin]

Allows to set set :puma_daemonize, true config to use Puma daemonize instruction.

This can be useful when not using Puma's --daemon option or through pumactl.

[seuros]

@rhannequin thank you for your contribution ? Do you know which version of puma started to support this option ?

[waltercool]

It looks like since Puma 2.0 https://github.com/puma/puma/blob/master/History.md#200b2--2012-11-18 , but I can only confirm the existence of this option since 2.12 https://github.com/puma/puma/blob/v2.12.0/lib/puma/rack/builder.rb

[seuros]

Thank you @waltercool
I will rebase this in the v3 branch.