Allow use of sidekiq_flags in capistrano task to start

What
=====

In the case where extra flags need to be set the capistrano task doesn't support that. 

For instance in my case the way we need to upstart sidekiq is

`sidekiq -r ./lib/sidekiq_server.rb` which loads some defaults.

The way I have been able to get around this for now is setting rails_env as 'production -r ./lib/sidekiq_server.rb' but that seems pretty hacky to me. 

Taking a note out of the puma-capistrano project they have a puma_flags that you can add additional flags to if need be.

How to test
=========

No real good test strategy except for having an app that requires something like -r