Standalone Sidekiq::Launcher

* Add Sidekiq::Launcher, extracted from Sidekiq::CLI.

  This is a step forward to run Sidekiq inside a process,
  making it more easily to integrate into existing process.

* Extract procline feature from Manager to Launcher.

  We don't want to touch the process name if we're integrating
  Sidekiq into existing process, so better not to put procline
  assignment in Manager, but in CLI where we launch standalone
  Sidekiq process.