added whenever_command_environment_variables option to capistrano v3 options

I need to pass some environment variables to whenever when I update the crontab.  These are basically custom variables, which I cannot pass as command-line arguments.

I tried what is described in [Capistrano V3 Integration](https://github.com/javan/whenever/#capistrano-v3-integration) in the README: 

    SSHKit.config.command_map[:whenever] = "bundle exec whenever"

However, it didn't work the way I needed it to.  This PR is a more elegant, reliable solution.