Refactor and simplify CLI and Env:

* Do not modify Env logger from CLI;
* Add verbose attribute to Env;
* Implement Env#verbose?;
* Remove Env#log_level and Env#log_level=;
* Refactor related specs, improve some wording.

lib/producer/core/cli.rb

@@ -36,7 +36,7 @@ module Producer
         @arguments = arguments.inject([]) do |m, e|
           case e
           when '-v'
-            env.log_level = Logger::INFO
+            env.verbose = true
           when '-n'
             env.dry_run = true
           else