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.
Merge pull request #664 from godfat/launcher

Standalone Sidekiq::Launcher