Allow environment to be set for unicorn and runit

This commit allows environment variables to be set for the unicorn process by using the env attribute in the runit resource.

It is specified by using the environment attribute in the unicorn resource, for example:

```ruby
application "some_app" do
  #...
  unicorn do
    environment(
      'VARIABLE' =>  'some-value'
    )
  end
end