Add an option to include the `environment` rake task.

With Rails, Resque requires loading the Rails environment task to have
access to your models, etc. (e.g. `QUEUE=* rake environment
resque:work`). However, Resque is often used without Rails (and even if
you are using Rails, you may not need/want to load the Rails
environment).

Additionally, some users might have already set up Resque to depend on
the environment task, e.g.:

task "resque:setup" => :environment

As such, the `environment` task is not automatically
included. However, you can now add this setting to your `deploy.rb` to
have capistrano-resque require the `environment` task:

set :resque_environment_task, true