Accept a debug config in the start method

This will make easier to start a debug environment per action.

Author: Rafael Mendonça França

lib/thor/base.rb

@@ -439,7 +439,7 @@ def start(given_args = ARGV, config = {})
         config[:shell] ||= Thor::Base.shell.new
         dispatch(nil, given_args.dup, nil, config)
       rescue Thor::Error => e
-        ENV['THOR_DEBUG'] == '1' ? (raise e) : config[:shell].error(e.message)
+        config[:debug] || ENV['THOR_DEBUG'] == '1' ? (raise e) : config[:shell].error(e.message)
         exit(1) if exit_on_failure?
       rescue Errno::EPIPE
         # This happens if a thor command is piped to something like `head`,

spec/base_spec.rb

@@ -256,6 +256,12 @@ def hello
       end
     end
 
+    it 'raises an error instead of rescuing if :debug option is given' do
+      expect do
+        MyScript.start %w[what], :debug => true
+      end.to raise_error(Thor::UndefinedCommandError, 'Could not find command "what" in "my_script" namespace.')
+    end
+
     it 'does not steal args' do
       args = %w[foo bar --force true]
       MyScript.start(args)