Refactor bin/clockwork and add pidfile option

Issue: I run clockwork under `bluepill` monitoring system and needed to have clockwork do two thing:

1. Run in foreground for the sake of bluepill.
2. Save off a pidfile so that bluepill has easy access to pid.

Solution:

1. Refactor the behavior out of bin/clockwork into lib/clockwork/cli.rb
  - Allowed me to move behavior into methods for encapsulation/re-use.
  - Enabled testing of optparsing behavior for `options[:pidfile]`
2. Add specs to verify optparsing behavior

I also noticed that newer versions of `Minitest` don't properly run the specs. I pinned `Minitest` to an older version of gem that runs properly.

I made these changes on my own fork and then tidied them up a little bit in case the upstream maintainers would like to include them in `clockwork` gem.

If these changes help and fit the code style of the project, great! :smile:.