Unicorn: allow for custom runit templates (unicorn zero-downtime)

This change, combined with the below custom template/config allowed for zero-downtime deployments with unicorn:

**recipes/my_app.rb**

```ruby
application 'unicorn_zero_downtime' do

  # ...

  unicorn do
    preload_app true
    runit_cookbook '<my-cookbook>'
    runit_options pid: '/path/to/unicorn.pid'

    before_fork <<-EOF
      old_pid = '/path/to/unicorn.pid.oldbin'
      if File.exists?(old_pid) and server.pid != old_pid
        begin
          Process.kill('QUIT', File.read(old_pid).to_i)
        rescue Errno::ENOENT, Errno::ESRCH
        end
      end
    EOF

    restart_command Proc.new do
      runit_service 'unicorn_zero_downtime' do
        action :usr2
        sv_timeout 30
      end
    end
  end
end
```

**templates/default/sv-unicorn-run.erb** (modified from [this gist][], thanks @czarneckid)

[this gist]: https://gist.github.com/czarneckid/4639793

https://gist.github.com/JeanMertz/8996796